If A says that his mother is the only daughter of B's mother, how is B related to A?

Maternal Uncle

The correct answer is Option (3) → Maternal Uncle

1. Analyze the statement

The core of the puzzle is: "Only daughter of B's mother."

• B's mother has an "only daughter."

• This daughter must be B's sister (if B is male) or B herself (if B is female).

2. Connect the relationship to A

A says: "His (A's) mother is that person (the only daughter of B's mother)."

• This means A's mother and B's sister (or B) are the same person.

• If A's mother is B's sister, then B is the brother of A's mother.

• The brother of one's mother is called a Maternal Uncle.

B is related to A as Maternal Uncle, making Option (3) the correct choice.