The correct option is B Genetic drift
Gene flow, also called gene migration, is the migration or transfer of genes from the gene pool of one population to another. Genetic drift is the variation in relative frequency of different genotypes in a small population, owing to the chance disappearance of particular genes, as individuals die or do not reproduce. This affects the gentic makeup of the population, but in a random manner unlike natural selection (where the fittest species survive). In population genetics, the founder effect is an example of genetic drift. Founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals arising from a larger population.