The correct option is C When the population live in different conditions Genetic variation is the naturally occurring genetic differences among individuals of the same species. This variation permits flexibility and survival of a population in the face of changing environmental circumstances.
Factors affecting the gene frequency of population:
Physical distribution of individuals affects a population.
When a population interbreeds.
Random fluctuations in the numbers of alleles in a population.
Migration is the movement from one location to another.
So the answer is 'When the population live in different conditions'.