Hardy-Weinberg equilibrium is a theoretical concept that explains the distribution of alleles in a population. It assumes that certain conditions are met, including:

1. No mutations: The gene pool of the population should not be altered by new mutations.

2. No migration: There should be no immigration or emigration of individuals between populations.

3. Large population size: The population should be sufficiently large to avoid the effects of genetic drift.

4. Random mating: Individuals should mate randomly, with no preference for certain traits or individuals.

If any of these conditions are not met, the population will not be in Hardy-Weinberg equilibrium. In particular, if individuals mate selectively, the frequency of alleles in the population will change from generation to generation, leading to a departure from equilibrium.

Selective mating can occur in various ways, including:

- Assortative mating: Individuals preferentially mate with others who have similar phenotypic traits, such as body size or coloration.

- Disassortative mating: Individuals preferentially mate with others who have dissimilar phenotypic traits.

- Inbreeding: Individuals mate with close relatives, which increases the frequency of homozygous genotypes and reduces genetic diversity.

- Sexual selection: Individuals preferentially mate with others who have certain desirable traits, such as bright plumage or elaborate courtship displays.

All of these forms of selective mating can alter the frequency of alleles in a population, which can lead to changes in phenotype and genotype frequencies over time. Therefore, selective mating is a major factor that can disrupt Hardy-Weinberg equilibrium.

Random mating means inviduals mate selectively.. so option "d" is correct..