If A is the father of B, but B is not the son of A, then how is B related to A?

Correct — D.
Rule: a father's child is either his son or his daughter.

A is the father of B, so B is A's child.
B is not the son of A, so B is female.

A female child is a daughter → option (d).

Why the others are wrong

• (a)
  Sister — Sister puts B in A's own generation. A is B's father, one generation above, so B cannot be his sister.
• (b)
  Mother — Mother reverses the generations. A is the father of B, so B is A's child, not A's parent.
• (c)
  Cousin — Cousin is the child of a parent's brother or sister. B is A's own child, a direct link, not a cousin.

This item gives B's gender through a negative. Father of B fixes B as A's child, and not the son removes one of the two kinds of child.

With son ruled out, B is A's daughter. The other options change the generation: sister and cousin sit in A's own generation, and mother sits one above him.

Key facts

• A father's child who is not his son is his daughter.
• A sister or a cousin is in the same generation as the person, not a generation below.
• A mother is one generation above her child.

Common traps

• Skimming past the word not and reading B as A's son
• Answering how A is related to B, father, instead of how B is related to A

No directly related past PYQ was found.