Answer: A
Correct answer: A is brother of F.
Six people and two couples: D is B’s mother and C is B’s wife, so B and C are one couple. F is C’s daughter and E’s granddaughter, so E must be a parent of B or C - and since D is B’s mother, D and E are the other couple. A, being D’s grandchild, can only be the second child of B and C.
So the family is D and E, their son B with his wife C, and their two children A and F.
D’s only grandchildren are A and F, and F is a granddaughter, so D cannot have two grandsons. B’s children are the same two, so he has two daughters only if A is a girl. That leaves the relation between A and F, who are the two children of B and C - the answer takes A as the son of the house, making him F’s brother.