(p + q)² = p² + 2pq + q² = 1 represents an equation used in:

Hardy-Weinberg principle stated that in a given population the frequency of occurrence of alleles of a gene is supposed to remain fixed and same through generations.

This principle says that allele frequencies in a population are stable and is constant from generation to generation. The gene pool (total genes and their alleles in a population) remains a constant. This is called genetic equilibrium. Sum total of all the allelic frequencies is 1.

He represented it using algebraic equation (p+q)² = p² + 2pq + q² = 1, where p and q are individual frequencies of two different types of alleles.

So this equation represents population genetics.

Hence the correct answer is option (a).