Ans. (C). Aythya affinis and Aythya marila
The scientific name of an organism consists two words, out of which the first word represents genus and the second word represents species of that organism.
According to the classification, there are seven levels of classification from least to most closely related, which include kingdom, phylum, class, order, family, genus, and species.
Organisms Aythya affinis and Aythya marila belong to a common genus (Aythya), and thus, they are most closely related to each other as compared to other given groups.
Thus, the correct answer is option (C).