B is the husband of P. Q is the only grandson of E, who is wife of D and mother-in-law of P. How is B related to D ?

The correct answer is C. Son-in-law.

B is the husband of P, who is the daughter of D. This means that B is the son-in-law of D.

A nephew is the son of a person’s brother or sister. A cousin is a person who shares one or more grandparents with another person. A son is a male child of a parent.

In this case, B is not related to D in any of these ways. He is not the son of D’s brother or sister, he does not share any grandparents with D, and he is not D’s male child. Therefore, the only possible answer is C.