Introducing a boy, a girl says, "He is the son of my mother’s only son." Who is the boy to the girl?

Correct — C.
Rule: a girl's mother's only son is her brother (she is a daughter, not a son).

The girl's mother's only son = the girl's brother.
The boy is that brother's son.

A brother's son is a nephew → option (c).

Why the others are wrong

• (a)
  Brother — Brother is the boy's father, not the boy. The girl's mother's only son is her brother, and the boy is one generation below him.
• (b)
  Son — Son would need the girl to be the boy's parent. His father is the girl's brother, so the boy is her brother's child, not her own.
• (d)
  Cousin — Cousin is the child of a parent's sibling. The boy is the child of the girl's own brother, which makes him a nephew.

'My mother's only son' points to a different person depending on who speaks. A man saying it means himself. A girl saying it means her brother, because she cannot be her mother's son.

Once the brother is fixed, the boy is one generation below the girl, on her brother's side: a nephew.

Key facts

• A brother's or sister's son is a nephew, and their daughter is a niece.
• A man's 'mother's only son' is the man himself.
• A woman's 'mother's only son' is her brother.

Common traps

• Reading 'my mother's only son' as the speaker, which is right for a man and wrong for a girl
• Answering Brother by stopping one step short, at the boy's father

No directly related past PYQ was found.