Pointing to a photograph, a person said, "He is the son of my mother's only…

Concept: In blood-relation puzzles, build a mini family tree from the stated links, always keeping the speaker as the reference point. A parent's sibling is the speaker's uncle/aunt; that sibling's child is the speaker's cousin (same generation as the speaker); the speaker's own sibling's child is the speaker's nephew/niece (one generation below the speaker).

1. The photograph is of "the son of my mother's only brother."

2. "My mother's only brother" is the speaker's maternal uncle.

3. The person in the photograph is the son of that maternal uncle.

4. By the rule above, the son of one's maternal uncle is one's cousin.

Cross-check: each near-miss label fails for a distinct reason —

• "Brother" would require sharing both parents with the speaker; nothing here shows that.

• "Nephew" applies to the son of the speaker's own sibling, not the son of the speaker's uncle.

• "Uncle" applies to the mother's brother himself, one generation above the person being described.

So the person in the photograph is the speaker's cousin.