$\text{Correct option:C}$ 

$\text{Solution:}$

$\text{Option (A)}$

$?$ Population explosion is rapid increase in population of a given area ,over a relatively small period of time .

$?$ Emigration is when individuals from one population go out to anor place, this leads to loss of certain genes in gene pool. Population size decreases

$\text{Option (B)}$

$?$ Mortality rate is number of individuals dying in a unit time ,it is thus also known as death rate.Population size decreases due to mortality rate. This tends to increase population size.

$\text{Option (C)}$

$?$ Natality rate is average number of new individuals produced by a population reproductively active in a unit time. It is also known as birth rate.

$\text{Option (D)}$

$?$ Immigration is when individuals from one population migrate to anor,this introduces new alleles in gene pool of a population. Population size increases.

$\text{Hence, correct option is (C)}$