Pointing to a girl, a man said, "She is the daughter of my mother’s only son." How is the girl related to the man?

Correct — A. Rule: "my mother's only son", said by a man, is the man himself.

The speaker is a man, so he is a son of his mother. If she has one son, it is him.

So the girl is the daughter of the speaker: she is the man's daughter, option (a).

Why the others are wrong

• (b)
  Niece — A niece would need her father to be the man's brother. "Only son" leaves the man's mother no other son, so the girl's father is the man himself.
• (c)
  Sister — A sister would be the daughter of the man's mother, not the daughter of her son. The statement puts the girl one generation below the man.
• (d)
  Cousin — A cousin is the child of a parent's sibling. The statement names the mother's son, who is the man himself, not a sibling of his mother.

A relation phrase can loop back to the speaker. "My mother's only son", said by a man, names the man himself: he is her son, and she has no other.

Substitute that, and the sentence becomes "She is my daughter".

The word only carries the item. Without it, "a son of my mother" could be the man's brother, and the girl could be his niece.

Key facts

• A man's mother's only son is the man himself.
• Without "only", the phrase could name his brother, which would make the girl a niece.
• The girl is one generation below the man.

Common traps

• Reading "my mother's only son" as a brother and choosing Niece.
• Ignoring that the speaker is a man, which is what makes him the only son.

No directly related past PYQ was found.