In random mating population in equilibrium, which of the following brings about a change in gene frequency in a non -directional manner

The correct option is D Random drift
Hardy-Weinberg principle states that the allele frequencies remain constant in successive generations in a population which does not undergo evolutionary change.

Factors which bring about change in gene frequencies are mutation, migration, gene recombination and genetic drift (random drift).

Mutations introduce new alleles in a population and thereby change the allele frequencies.

Random drift also known as genetic drift is a change in the frequency of alleles in a non-directional manner. Such non-directional changes in gene frequencies cannot be predicted from generation to generation and occur in small populations.

Natural selection occurs due to increased prevalence of favorable traits and genes, and is directional.

Migration involves directional movement of organisms between different populations or geographical locations.

Hence the correct option is
b. Random drift