Answer:
The correct answer would be an option a.
Natural selection and genetic drift are the natural phenomenons which cause evolution.
They change the frequency of the allele(s) in a population.
The effect of genetic drift is maximum in small populations or when population size gets reduced to very small due to factors such as overhunting, loss of habitat, natural calamity et cetera.
Natural selection favors the survival and reproduction of better-adapted organisms and thus, it results in a change in the genotype frequency of the population.
