Assertion :Genetic drift refers to change in allelic frequencies of a gene pool due to chance and occurs both in large and small populations. Reason: Small populations will, therefore, suffer more than larger ones.

The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
Genetic drift is one of the basic mechanisms of evolution. In each generation, some individuals may, just by chance, leave behind a few more descendants (and genes) than other individuals. The genes of the next generation will be the genes of the lucky individuals, not necessarily the healthier or better individuals. Genetic drift or allelic drift is the change in the frequency of a gene variant (allele) in a population due to random sampling. The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces. A population's allele frequency is the fraction of the copies of one gene that share a particular form. Genetic drift may cause gene variants to disappear completely and thereby reduce genetic variation. Genetic drift occurs in both small and larger populations and small populations will, therefore, suffer more than larger ones. So, both Assertion and Reason are correct and Reason is the correct explanation for Assertion.