Answer:
c. Unusually high migration variance Individuals that are very successful at migrating to new populations, but cannot mate in the new locations.
Step-by-step explanation:
Gene flow refers to the movement of alleles between the local populations when individuals form a population migrate into the other population. Gene flow can occur only if the migrated individuals mate successfully with the individuals of the other population. This would add new alleles from immigrant individuals to the gene pool of the population leading to evolution.
If the immigration is not followed by the mating of the immigrants with individuals of the local population, there would not be any addition of new alleles to the gene pool and the population would not exhibit evolution by gene flow.