Pointing to a photograph Lata says, "He is the son of the only son of my…

Concept

In a blood-relation chain, decode the sentence from the innermost phrase outward, one generation at a time. Each phrase ("son of", "only son of", "grandfather") maps to a single link in the family tree. A crucial convention in these puzzles: when "grandfather" is stated without qualification together with "the only son", it denotes the speaker's grandfather along the direct line whose only son is the speaker's own father.

Applying it

1. Start with the innermost link: "my grandfather" is Lata's grandfather.

2. "the only son of my grandfather" - since the grandfather has exactly one son, that son is Lata's father.

3. "the son of ... (Lata's father)" - a son of Lata's father is a male child sharing Lata's parents.

4. A son of Lata's father is a sibling of Lata; since the photograph shows a man, that sibling is Lata's brother.

So the man in the photograph is Lata's brother.

Cross-check and the maternal reading

Could "grandfather" instead be the mother's father? In that reading the only son would be Lata's maternal uncle and his son would be a cousin. Standard reasoning convention resolves "the only son of my grandfather" along the paternal line - it equals "my father" - so the intended and accepted answer is brother. The cousin reading is the trap built for the maternal interpretation; it is not the conventional answer to this stem.