Population density means

$A$ population is a group of all . . . . . . in a given time.

Define the following terms:
$(1)$ Immigration
$(2)$ Emigration
$(3)$ Mortality

Define 'zero population growth rate'. Draw an age pyramid for the same.

Read the following statements and select the correct option with respect to population attributes:
$(a)$ Population density is necessarily measured in numbers.
$(b)$ Tiger census is often based on pug marks and fecal pellets.
$(c)$ Biomass is not a meaningful measure to know population size.
$(d)$ Size of a population for any species is not a static parameter.

Consider the birth rate and death rate per year in a given population.
Total population after $5$ years $\rightarrow 100$
Number of individuals dying per year $\rightarrow 10$
Number of individuals born per year $\rightarrow 20$