The correct answer is option A) decreased birth rate.
The decrease in the birth rate would lead to overall decrease in the population, as the number of new individuals added to the population would be less.
Reason for the incorrect options:
An increased in the immigration of individuals, would increase the overall of number of individuals in a particular area, so, it would lead to increase in population.
The decreased infant mortality or overall mortality would allow the survival of more individuals in a particular region, which would cause an increase in the population.