AIIMS 2007 Biology Question Paper with Answer and Solution

(D) $Neurospora$ belongs to the class $Ascomycetes$,in which ascospores are produced in sac-like structures called $asci$.
$Neurospora$ is commonly known as the pink or red bread mould.
It is widely used in genetic experiments because it can be grown easily in laboratory conditions.
Water moulds are typically members of the class $Oomycetes$ (e.g.,$Phytophthora$ or $Saprolegnia$),not $Neurospora$.
Therefore,both the Assertion and the Reason are incorrect.

(A) The coralloid roots of $Cycas$ contain a specialized region known as the algal zone.
This zone is inhabited by symbiotic blue-green algae (cyanobacteria),such as $Anabaena$ and $Nostoc$.
This is a mutualistic relationship where the algae receive shelter and nutrients from the $Cycas$ plant,while the $Cycas$ plant benefits from the nitrogen-fixing capabilities of the algae.

(C) Nymphaea,Sagittaria,Ceratophyllum,and Vallisneria are hydrophytes that grow in water.
Nymphaea,Sagittaria,and Vallisneria are rooted hydrophytes,meaning they are anchored in the mud.
Ceratophyllum is a submerged,free-floating hydrophyte.
It remains completely underwater but is not rooted in the mud.
Ceratophyllum lacks roots even during its embryonic stage.

(C) Thallophytes are plants whose body is not differentiated into root,stem,and leaves.
Algae and fungi both possess a thalloid plant body,hence they are classified as thallophytes.
Algae are autotrophs,meaning they can synthesize their own food through photosynthesis.
Fungi are heterotrophs,meaning they cannot synthesize their own food and depend on other organic sources.
Therefore,the Assertion is correct,but the Reason is incorrect.

(A) In $Pinus$, a conifer tree, microspores are produced by microsporogenesis in the microsporangium.
Each microsporangium has an inner nourishing layer known as the tapetum.
$A$ large number of dusty and two-winged microspores (pollen grains) are produced.
On maturity, the microsporangium wall bursts, and microspores are released into the air, which is often referred to as a "shower of sulphur."
These pollen grains are dispersed by wind specifically due to the presence of wings, which provide buoyancy.
Therefore, the presence of wings is the reason why they are efficiently wind-borne in large quantities.

(D) lamprey (sometimes also called as lamprey eel) is a jawless fish that belongs to the class $Cyclostomata$.
Lampreys possess a toothed,funnel-like sucking mouth and have scaleless,slimy skin with seven pairs of circular gill slits on each side.
They are well known as ectoparasites that bore into the flesh of other fish to suck their blood.
In contrast,Flying fox $(Pteropus)$,Hedgehog $(Erinaceus)$,and Manatee $(Trichechus)$ are all members of the class $Mammalia$.

(B) Ctenidium is a comb-like respiratory organ found in mollusks,particularly gastropods.
During the larval development of gastropods,a process called $Torsion$ occurs,which involves a $180^{\circ}$ rotation of the visceral mass,mantle,and shell.
As a result of this torsion,the ctenidium (along with the anus and mantle cavity) shifts from its original posterior position to an anterior position.
Therefore,the assertion that torsion is seen in the ctenidium is correct.
The reason that the ctenidium acts as a respiratory organ is also a correct biological fact.
However,the process of torsion is a developmental phenomenon and is not caused by the fact that the ctenidium functions as a respiratory organ.
Thus,both statements are correct,but the reason is not the correct explanation for the assertion.

(C) Orchids are epiphytes that possess aerial roots. These roots contain a specialized spongy tissue called velamen. The primary function of velamen is to absorb moisture from the atmosphere or air,which is essential for the survival of these plants as they do not grow in soil.

(B) Composite fruits,also known as multiple fruits,develop from the entire inflorescence rather than a single flower.
These are primarily of $2$ types: sorosis and syconus.
$1$. Sorosis develops from spike,spadix,or catkin inflorescences (e.g.,$Ananas$,$Artocarpus$).
$2$. Syconus develops from hypanthodium inflorescence (e.g.,$Ficus$).

(A) In woody trees,the central portion of the stem is dark in color. It is hard and tough due to the deposition of resins,tannins,gums,and the formation of tyloses. This central hard portion is called heartwood. It is formed by secondary growth. Due to cambial activity,the older secondary xylem becomes non-functional and forms heartwood or duramen. As the tree grows,more secondary xylem is produced,and the older layers are converted into heartwood,causing its amount to increase year after year. Therefore,both the Assertion and the Reason are correct,and the Reason is the correct explanation of the Assertion.

(D) Endoplasmic reticulum $(ER)$ is a network of membranous tubules and sacs present in the cytoplasm.
Rough $ER$ is studded with ribosomes and is primarily responsible for protein synthesis and the transport of enzymes.
Additionally, the $ER$ provides mechanical support to the cytoplasm, acting as a structural framework or cytoskeleton.

(A) The cell is the basic structural and functional unit of living organisms.
In $1855$, Rudolf Virchow explained that all cells arise from pre-existing cells through cell division, a concept he summarized as "Omnis-cellula-e-cellula".
Robert Hooke was the first to discover the basic units of life and termed them "cells".
Anton van Leeuwenhoek was the first to observe living unicellular organisms, including bacteria.
Robert Brown described the nucleus as a characteristic spherical body found in plant cells.

(C) Lactose,also known as milk sugar,is found exclusively in milk and contains a $\beta-1,4$ glycosidic linkage.
It is a disaccharide formed by the condensation reaction between a molecule of galactose and a molecule of glucose.

(A) Nucleotides consist of carbon,hydrogen,oxygen,nitrogen,and phosphorus.
Nitrogenous bases are classified as purines or pyrimidines.
Purines (Adenine and Guanine) are $9$-membered double-ringed structures containing $4$ nitrogen atoms.
Pyrimidines (Cytosine,Thymine,and Uracil) are single-ringed structures.
Specifically,Cytosine has $3$ nitrogen atoms,while Uracil and Thymine have $2$ nitrogen atoms.
Therefore,the statement that Adenine has $4$ nitrogen atoms is correct.

(D) Vegetable oils and fats are present in plant cells in an insoluble form,not a soluble form. Therefore,the Assertion is incorrect.
Vegetable oils are not restricted to the embryo; they are found in various parts of plants such as seeds,fruits (e.g.,olive and palm oil from fleshy pericarp),roots,stems,and leaves. Therefore,the Reason is also incorrect.
Since both the Assertion and the Reason are false,the correct option is $D$.

(D) The upward movement of water and dissolved minerals (sap) through the xylem against the force of gravity is known as the ascent of sap.
Root pressure provides a positive push to the water column.
Transpiration creates a negative pressure (transpiration pull) that pulls water upwards.
Capillarity (capillary action) due to the narrow diameter of xylem vessels also assists in the movement of water.
Therefore,all these factors contribute to the ascent of sap.

(C) Cultivation of plants by placing the roots in a nutrient solution without any soil is called hydroponics.
It is also known as soilless culture,water culture,or solution culture.
This technique is used to determine which mineral elements are essential for plant growth and to observe the symptoms produced by the absence or deficiency of these essential elements.

(B) The root nodules of leguminous plants contain symbiotic nitrogen-fixing bacteria called $Rhizobium$.
Root nodules are small,irregular outgrowths on the roots which appear pinkish internally due to the presence of a pigment called leghaemoglobin.
This pigment is structurally related to the blood pigment haemoglobin.
Leghaemoglobin acts as an oxygen scavenger,which creates an anaerobic environment by binding to oxygen.
This is crucial because the nitrogen-fixing enzyme,nitrogenase,is highly sensitive to molecular oxygen $(O_2)$ and is inactivated in its presence.
Therefore,leghaemoglobin protects nitrogenase from $O_2$ to ensure efficient nitrogen fixation.

(B) Essential elements are classified into macronutrients and micronutrients based on the quantity required by plants.
Micronutrients (trace elements) are required in very small amounts (less than $10 \text{ mmol kg}^{-1}$ of dry matter).
The micronutrients include Iron $(Fe)$, Manganese $(Mn)$, Copper $(Cu)$, Molybdenum $(Mo)$, Zinc $(Zn)$, Boron $(B)$, Chlorine $(Cl)$, and Nickel $(Ni)$.
Macronutrients are required in larger amounts and include Carbon $(C)$, Hydrogen $(H)$, Oxygen $(O)$, Nitrogen $(N)$, Phosphorus $(P)$, Sulfur $(S)$, Potassium $(K)$, Calcium $(Ca)$, and Magnesium $(Mg)$.
Option $B$ contains $Cu, Fe, Zn, B, Mn$, all of which are micronutrients.

(A) Sulphur is an essential element and a constituent of amino acids like $Cysteine$,$Cystine$,and $Methionine$.
Sulphur is present in the soil primarily in the form of organic matter or inorganic compounds.
Sulphur-oxidizing bacteria (such as $Thiobacillus$) are required to convert these forms into $SO_4^{2-}$ (sulphate ions).
Plants then absorb sulphur from the soil specifically in the form of $SO_4^{2-}$ ions.
Therefore,both the Assertion and the Reason are correct,and the Reason provides the correct explanation for the Assertion.

(B) $PAR$ stands for Photosynthetically Active Radiation.
It refers to the spectral range of solar radiation from $400\, nm$ to $700\, nm$ that photosynthetic organisms are able to use in the process of photosynthesis.
Therefore,the correct range is $400\, nm - 700\, nm$.

(C) The dark reaction of photosynthesis is a series of enzymatic reactions that reduce $CO_2$ into carbohydrates using the assimilatory power ($ATP$ and $NADPH_2$) generated during the light reaction.
Since these reactions do not directly require light energy to proceed,they are termed 'dark reactions' or 'light-independent reactions'.
However,the term 'dark reaction' is a misnomer because these reactions can occur in the presence of light as well,provided the necessary $ATP$ and $NADPH_2$ are available.
Therefore,the assertion that the dark reaction is purely enzymatic is correct,but the reason that it occurs only in the absence of light is incorrect.

(A) Acetyl $CoA$ acts as the connecting link between glycolysis and the Krebs cycle. During the transition reaction,pyruvic acid is converted into Acetyl $CoA$. Subsequently,Acetyl $CoA$ combines with oxaloacetic acid to form citric acid,which marks the initiation of the Krebs cycle.

(C) Retinol is the chemical name of vitamin $A$,which is primarily found in carrots,milk,cheese,and fish.
Retinol is essential for light absorption in animals,where it is converted into the light-absorbing molecule called retinal.
Deficiency of vitamin $A$ affects the rod cells in the retina,leading to a condition known as night blindness.
Tocopherol is the chemical name of vitamin $E$,which is found in wheat germ oil and brown flour; its deficiency causes sterility in rats.
Vitamin $C$,or ascorbic acid,is found in citrus fruits and potatoes; its deficiency causes scurvy,which affects joints and gums.

(D) Sugarcane and sugarbeet are the richest sources of sucrose,which is a disaccharide.
In the human digestive system,the enzyme invertase (also known as sucrase) is present in the duodenum of the small intestine.
This enzyme hydrolyzes sucrose into one molecule of glucose and one molecule of fructose.
Option $A$ is incorrect because pepsin acts on proteins.
Option $B$ is incorrect because salivary amylase acts on starch to produce maltose,not glucose.
Option $C$ is incorrect because lipases act on triglycerides to produce fatty acids and monoglycerides.
Therefore,option $D$ correctly describes the enzymatic breakdown of sucrose.

(D) Blood contains an anticoagulant called $heparin$,which prevents the activation of $prothrombin$.
$Prothrombin$ is an inactive plasma protein required during the process of blood coagulation.
$Heparin$ is a natural anticoagulant released from the mast-cell granules that prevents blood from clotting inside the intact blood vessels.

(A) high concentration of cholesterol in the blood leads to the deposition of cholesterol on the inner walls of the arteries.
This process causes the arteries to lose their elasticity and become stiffened.
This condition is medically termed as arteriosclerosis,which literally means the hardening of the arteries.

(D) The Assertion $(A)$ is incorrect because blood does not coagulate in uninjured blood vessels under normal physiological conditions.
The Reason $(R)$ is correct because the endothelial lining of blood vessels secretes heparin,which is a natural anticoagulant that prevents the clotting of blood within the circulatory system.
Therefore,since the Assertion is false and the Reason is true,the correct option is $(D)$.

(A) The Basal Metabolic Rate $(BMR)$ is defined as the energy requirement of the body at rest. Smaller organisms have a higher surface-area-to-volume ratio,leading to greater heat loss and consequently a higher $BMR$ per gram of body weight to maintain homeostasis.
Metabolic rate is directly correlated with physiological parameters like heart rate. $A$ six-month-old baby has a significantly higher metabolic demand and a faster heart rate ($70-190$ beats/minute) compared to an elderly person ($60-100$ beats/minute).
Therefore,both statements are correct,and the higher heart rate in a baby serves as a physiological example supporting the higher metabolic rate observed in smaller organisms.

(D) Hypotonic urine refers to urine that has a lower solute concentration than blood plasma.
Hypertonic urine,on the other hand,is more concentrated than blood and possesses a higher osmotic pressure than the blood.
Producing hypertonic urine is the mechanism that helps in reducing the loss of water through urine.
Since the Assertion claims that hypotonic urine reduces water loss (which is false) and the Reason incorrectly defines hypotonic urine as more concentrated than blood (which is also false),both statements are incorrect.

(A) Aldosterone is a steroid hormone (mineralocorticoid) secreted by the adrenal cortex.
It plays a crucial role in maintaining electrolyte balance,specifically sodium $(Na^+)$ and potassium $(K^+)$ ions in mammals.
It acts on the distal convoluted tubules and collecting ducts of the nephron to promote the reabsorption of sodium ions and water,while simultaneously promoting the excretion of potassium ions.
This process increases the sodium ion concentration in the extracellular fluid $(ECF)$ and blood,thereby regulating blood pressure and blood volume.
Therefore,both the Assertion and the Reason are correct,and the Reason provides a correct explanation for the Assertion.

(D) The shoulder blade is known as the $scapula$.
It is a large,flat,triangular bone situated in the dorsal part of the thorax between the $2^{nd}$ and the $7^{th}$ ribs.
It forms a part of the pectoral girdle.
The $scapula$ articulates with the $clavicle$ (collar bone) anteriorly and the $humerus$ (arm bone) laterally.

(C) The forebrain is the largest part of the brain,consisting of two halves called cerebral hemispheres,which are separated by longitudinal fissures.
Each cerebral hemisphere is divided into four lobes: the frontal lobe,parietal lobe,temporal lobe,and occipital lobe.
The temporal lobe contains the auditory cortex,which is responsible for processing sensory information related to hearing and smell.
Therefore,hearing impairment is associated with the temporal lobe.
The frontal lobe is concerned with voluntary movements and personality.
The parietal lobe processes general sensations like temperature,touch,pressure,pain,and proprioception.
The occipital lobe contains centers for visual sense.

(B) The middle layer of the eyeball is the choroid,which contains many blood vessels and looks bluish in color. The choroid layer is thin over the posterior two-thirds of the eyeball,but it becomes thick in the anterior part to form the ciliary body. This layer contains numerous melanocytes that produce black pigment,which absorbs light and prevents internal reflection within the eyeball.

(A) Astigmatism is a vision defect where the cornea or lens has an irregular curvature,causing distorted or blurred vision because light rays do not focus at a single point on the retina.
Since the curvature is uneven,the eye cannot focus light equally in all meridians.
Cylindrical lenses are specifically designed to compensate for this uneven curvature by providing different refractive powers in different meridians.
Therefore,both the Assertion and the Reason are correct,and the Reason correctly explains why cylindrical lenses are used to treat astigmatism.

(A) Oxytocin is a hormone secreted by the posterior pituitary that causes contraction of the smooth muscles of the myometrium during childbirth and the ejection of milk from the mammary glands.
Glucagon is secreted by the $\alpha$-cells of the Islets of Langerhans in the pancreas. Its main function is to increase blood glucose levels.
Adrenaline (epinephrine) is secreted by the adrenal medulla. It increases the rate and force of the heartbeat.
The thyroid gland secretes thyroxine,which regulates the basal metabolic rate $(BMR)$.

(C) The pancreas is a heterocrine gland,meaning it functions as both an endocrine and an exocrine gland.
Its endocrine portion contains clusters of cells known as the islets of Langerhans.
The $\alpha -$ cells of the islets of Langerhans secrete glucagon,while the $\beta -$ cells secrete insulin.
Glucagon and insulin have antagonistic effects on blood glucose levels.
Insulin acts to decrease blood glucose levels,whereas glucagon acts to increase blood glucose levels by promoting glycogenolysis and gluconeogenesis.
Therefore,the statement that glucagon decreases blood sugar level is false.

(B) "Homo sapiens" refers to human beings.
Humans belong to an order of mammals called primates and are placed in the family $Hominidae$.
$Hominidae$ includes both modern and archaic human beings and also consists of Neanderthals, which are considered a subspecies of $Homo$ $sapiens$.
Humans and their ancestors are more closely related to the apes.

(C) Cocoa is obtained from the seeds of the cocoa plant, $Theobroma$ $cacao$. It belongs to the family $Sterculiaceae$. This plant is a native of tropical America, and its seeds are fermented, dried, and roasted to prepare cocoa and chocolate.

(B) Vernalization is a process where the flowering of plants is promoted or accelerated by exposing them to a period of low temperature.
This process ensures that plants flower during a favorable season.
The term 'vernalization' was coined by $T.D.$ Lysenko in $1928$.
It is believed that a hypothetical hormone called 'vernalin' is produced in the plant as a result of this cold treatment,which triggers flowering.
The perception of the cold stimulus occurs primarily in the actively dividing cells,specifically the apical meristem of the shoot or the embryo.
Since both the Assertion and the Reason are scientifically accurate,but the Reason explains the location of the process rather than the mechanism of acceleration,the correct choice is $B$.

(B) Citrus canker is a plant disease caused by the bacterium $Xanthomonas$ $citri$.
$Wilt$ $disease$ is typically caused by $Fusarium$ $oxysporum$.
$Red$ $rot$ $of$ $sugarcane$ is caused by the fungus $Colletotrichum$ $falcatum$.
$Powdery$ $mildew$ is caused by various fungi such as $Erysiphe$ or $Oidium$.

(A) Petroplants are plants that produce a large amount of latex containing long-chain hydrocarbons.
These hydrocarbons can be processed into liquid fuels,making them a potential substitute for petroleum.
The production of latex is a metabolic adaptation in these plants,which allows them to store energy in the form of hydrocarbons.
Therefore,the reason correctly explains why these plants are classified as petroplants.
Examples include $Euphorbia$ $antisyphilitica$,$Euphorbia$ $lathyris$,and $Calotropis$ $procera$.

(B) Reproduction is the biological process by which new individual organisms are produced from their parents.
In $Euglena$,binary fission occurs longitudinally.
In $Paramecium$,binary fission occurs transversely.
$Amoeba$ typically reproduces by simple binary fission,but under unfavorable conditions,it can undergo multiple fission.
$Plasmodium$ reproduces by multiple fission.
Therefore,the correct match is $Paramecium \to$ transverse binary fission.

(B) In plants,normal sexual reproduction includes meiosis and fertilization. It is called amphimixis.
But in some plants,an abnormal form of reproduction called apomixis has been observed.
Apomixis is a form of asexual reproduction that mimics sexual reproduction,where seeds are formed without the processes of syngamy (fertilization) and meiosis.
In this process,the egg or other cells like synergids or antipodals develop into an embryo without fertilization.
The term apomixis was coined by Winkler $(1908)$.
Examples include $Citrus$ and $Ranunculus$.

(B) Sperm is the male gamete that fuses with the female gamete to produce a diploid cell called a zygote.
During fertilization,the acrosome of the sperm releases enzymes,particularly hyaluronidase,which facilitates the penetration of the sperm into the ovum.
Hyaluronidase acts on the hyaluronic acid of the corona radiata,allowing the sperm to reach the zona pellucida.
Acrosin is a protease that helps in the digestion of the zona pellucida,not the corona radiata.
Capacitation is the physiological maturation of sperm that occurs in the female reproductive tract,not the penis.

(A) When one gene masks the effect or activity of another gene which does not lie on the same locus,it is called epistasis. The gene that suppresses the other is known as the epistatic gene,while the gene whose expression is masked is called the hypostatic gene. Epistasis refers to non-allelic interactions. For example,coat colour in mice is controlled by an epistatic gene. When a coloured $(CCaa)$ mouse is crossed with an albino $(ccAA)$ mouse,agouti mice $(CcAa)$ appear in the $F_1$ generation. In the $F_2$ generation,agouti,coloured,and albino mice are obtained in a $9:3:4$ ratio.

(A) Turner's syndrome is caused due to the absence of one $X$ chromosome.
The genotype is $XO$ instead of the normal $XX$,resulting in a total of $45$ chromosomes instead of the normal $46$.
Individuals with this condition are phenotypically females who are sterile as they lack functional ovaries.
Other characteristics include short stature and underdeveloped secondary sexual characteristics.

(C) Haemophilia,also known as bleeder's disease,is a classic example of recessive sex-linked inheritance in humans.
It is masked in the heterozygous condition.
The person suffering from this disease lacks clotting factors $VIII$ and $IX$,which are essential for blood coagulation.
$A$ small cut may lead to continuous bleeding.
Men are primarily affected by this disease,while women typically act as carriers.
The mutation of a structural gene on chromosome number $15$ is associated with Marfan syndrome,not haemophilia.
Marfan syndrome results in the formation of abnormal connective tissues and extreme looseness of joints.
Therefore,the Assertion is correct,but the Reason is incorrect.

(C) Wings of pigeons (birds),mosquitoes (insects),and bats (mammals) are analogous organs,meaning they perform the same function (flight) but have different anatomical structures and embryonic origins.
This similarity in function,which has developed in distantly related groups,is an adaptation to the same environmental pressure (the need for flight).
Therefore,this phenomenon is known as analogy or convergent evolution.

(C) $Eohippus$ is the earliest known fossil form in the phylogeny of the horse.
It originated during the $Eocene$ epoch, not the $Pliocene$ epoch.
The first fossil of the horse was discovered in North America and was named $Eohippus$ or "Dawn Horse," which was later renamed as $Hyracotherium$.
Therefore, the Assertion is correct, but the Reason is incorrect.

(B) Antigens are substances that,when introduced into the body,stimulate the production of antibodies.
They are mostly proteins but can also be carbohydrates,lipids,or nucleic acids.
Every antigen possesses specific sites called antigenic determinants or epitopes,which allow the immune system to recognize them.
The recognition ability of the immune system (specifically innate immunity) is present from birth and does not require prior exposure to the antigen.
Therefore,both the Assertion and the Reason are correct,but the Reason does not explain why antigens have antigenic determinants; it describes the nature of the immune system's recognition capability.
Thus,the correct option is $B$.

(C) Tropical rain forests have a high density of organisms.
Tropical rain forests are mainly found in Central America,South America,the Congo River basin of Africa,and South East Asia.
In this biome,rainfall and warmth are abundant,leading to luxuriant plant growth.
This biome possesses more than half of the world's flora and fauna.
Productivity is very high,and the diversity of life is so immense that a single hectare of forest may contain $200$ species of trees.
Approximately $70-80\%$ of all insects and $80-85\%$ of all birds are known to inhabit tropical rain forests.

(B) Ecological diversity is categorized into three types: $\alpha$, $\beta$, and $\gamma$ diversity.
$\alpha$-diversity refers to the diversity within a single community or habitat.
$\beta$-diversity refers to the diversity that exists between communities or habitats, often representing the change in species composition along an environmental gradient.
$\gamma$-diversity refers to the total diversity of a landscape or geographic region.
Therefore, $\beta$-diversity is the diversity between communities.

(C) The stratosphere contains the ozone layer,which is primarily located between $15\, km$ and $30\, km$ above the Earth's surface.
This layer is often referred to as the ozonosphere.
Ozone molecules in this region absorb harmful ultraviolet $(UV)$ radiation from the sun,protecting life on Earth.
The temperature in the stratosphere increases with altitude due to the absorption of $UV$ radiation by ozone.
Chemical reactions involving nitrogen oxides and hydrocarbons in the presence of $UV$ rays lead to the formation of ozone $(O_3)$ and other pollutants like peroxyacetyl nitrate $(PAN)$.

(D) Pollution is defined as an undesirable change in the physical,chemical,or biological characteristics of air,water,and land that causes harmful effects on living organisms.
Pollution can be natural or anthropogenic (man-made). Natural sources of pollution include volcanic eruptions,forest fires,and soil erosion.
Therefore,the Assertion $(A)$ is incorrect because pollution is not always caused by human activities.
Contamination refers to the presence of harmful substances or organisms in an environment,whereas pollution implies that these substances cause adverse effects on the ecosystem.
Thus,pollution is distinct from contamination,making the Reason $(R)$ incorrect.
Since both the Assertion and the Reason are incorrect,the correct option is $(D)$.

(C) The stratosphere of the Earth's atmosphere contains an ozone layer that protects us from harmful ultraviolet $(UV)$ radiations from the sun.
Chlorofluorocarbons $(CFCs)$,which contain chlorine,are used as coolants in air conditioners and refrigerators and are the primary chemicals responsible for ozone depletion.
After being released into the troposphere,$CFCs$ migrate to the stratosphere,where they are broken down by $UV$ radiation,releasing chlorine atoms.
In the presence of sunlight,these chlorine atoms break down $O_3$ into $O_2$.
While the Assertion is correct,the statement that the ozone level decreases by $67\%$ every year is factually incorrect,as the depletion rate is not a fixed annual percentage of $67\%$ globally.
Therefore,the Assertion is correct,but the Reason is incorrect.