Answer:
C. Population shrinks; 15 birds in the end
Explanation:
Let us understand some of the given terms;
immigration is a form of migration in which organisms comes into a territory.
emigration is a form of migration in which organisms leaves a place
population is a group of organisms of the same species
Given:
Population = 21 birds
Immigrant = 3 birds
Emigrants = 9 birds
Population change = total number of birds + immigrant - emigrants
= 21 + 3 - 9
= 15 birds
The population will shrink giving 15 birds in the end