Pointing a photograph, A said to his friend B, "She is the only daughter of…

To determine how A is related to the person in the photograph, let's break down the statement "She is the only daughter of the father of my mother."

Step-by-Step Analysis
Analyze the inner phrase: "the father of my mother"

This person is A's grandfather (specifically, maternal grandfather).

Analyze the full phrase: "the only daughter of [the father of my mother]"

The only daughter of A's maternal grandfather would be A's mother (assuming there is only one daughter).

Combine the relationships:

A points to the photograph and says, "She is [my mother]."

The question asks: "How is A related to the person in the photograph?"

Since the person in the photograph is A's mother, A must be the son (or daughter, but given the options and the phrasing "his daughter" in the feedback, "Son" is the intended answer).