​If A is the father of B, who is the father of C, and D is the only son of C and C's wife F, then how is D related to A?

A is the father of B.

B is the father of C.

D is the only son of C and C's wife, F.

From the above following family tree will be - 

D is the great-grandson of A.

Thus, option (A) is right.