A says to B, “I am the daughter of the only son of your father.” How is B related to A?

Correct — B.
Rule: said to a man, 'the only son of your father' is that man himself.

All four options are male relations, so B is a man.
B is his father's son, and his father has no other son, so that son is B.

A is the daughter of that son, so A is B's daughter.
B is A's father → option (b).

Why the others are wrong

• (a)
  Grandfather — Grandfather is B's father's relation to A. B himself is one generation lower: he is the only son, and A is his daughter.
• (c)
  Uncle — An uncle would need B's father to have two sons, B and A's father. 'Only son' rules that out, so the son is B himself.
• (d)
  Brother — A brother would share A's generation. A is the daughter of the only son, B, so B is one generation above her.

Blood-relation statements are solved by drawing the chain from the far end. Start at B's father, go down to his only son, then to that son's daughter, who is A.

The word only does the work. A man who is his father's only son is himself the son being described, so 'the only son of your father', said to a man, means 'you'.

Key facts

• Said to a man, 'your father's only son' is the man himself.
• Said to a woman, 'your father's only son' is her brother.
• A man's daughter is one generation below him: he is her father.

Common traps

• Treating 'your father's only son' as a third person, which leads to uncle
• Stopping at B's father and answering grandfather

No directly related past PYQ was found.