Hardly–weinberg law is related to –: (A) Population genetics, (B) Health economics, (C) Social medicine, (D) None of the above
Answer is A. Hardly-Weinberg law 

The hardly-Weinberg law states that "The relative frequencies of each gene allele tends to remain constant from generation to generation".
Thus, the study of gene frequencies, and the influences which operate to alter the "gene pool" and their long term consequences is the central theme in population genetics.