Which of the following compounds has the highest $pK_a$ value?

In the presence of a small amount of phosphorus,aliphatic carboxylic acids react with chlorine or bromine to yield a compound in which $\alpha$-hydrogen has been replaced by a halogen. This reaction is known as:

How many of the following compounds react with $NaHCO_3$ to evolve $CO_2$ gas?
$I$: $3$-Nitrophenol
$II$: $2,4$-Dinitrophenol
$III$: $4$-Hydroxybenzoic acid
$IV$: $4$-Methylbenzoic acid
$V$: $4$-Chlorophenol
$VI$: $2,4$-Dinitrotoluene
$VII$: $4$-Hydroxybenzenesulfonic acid
$VIII$: $4$-Sulfobenzoic acid
$IX$: Squaric acid
$X$: Maleic acid

$C_2H_5OH$ reacts with which of the following to produce a sweet,fruity smell?

Which of the following orders of acidic strength is incorrect?

An optically active compound $X$ has molecular formula $C_4H_8O_3$. It evolves $CO_2$ with $NaHCO_3$. $X$ reacts with $LiAlH_4$ to give an achiral compound. $X$ is