D, the son-in-law of B , is the brother-in-law of A who is the brother of C. How is A related to B ?

The correct answer is: Data inadequate.

We know that D is the son-in-law of B, and that D is the brother-in-law of A. We also know that A is the brother of C. However, we do not know the relationship between B and C. It is possible that B is the father of C, in which case A would be B’s son. However, it is also possible that B is the mother of C, in which case A would be B’s daughter. It is also possible that B is not related to C at all. Therefore, the answer to the question “How is A related to B?” is “Data inadequate.”