Calculate the population. There is a flock
of 21 birds. 3 birds immigrate to the flock
but 9 emigrate.

A. Population grows; 15 birds in
the end
B. Population grows, 27 birds in
the end
C. Population shrinks; 15 birds in
the end
D. Population shrinks; 27 birds in
the end

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

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).