Explanation:

- Let's break down Rahul's statement: "The son of his son is the father of my mother's daughter."

- "His son" refers to the man's son.

- "The son of his son" refers to the man's grandson.

- This grandson is "the father of my mother's daughter," who is essentially Rahul himself.

- Therefore, the man is Rahul's great-grandfather. This is because the man is the father of Rahul's grandfather.

Now, let's examine the options:

1. Grandpa: This would imply the man is Rahul's grandfather, which the statement does not support.

2. Father's father's brother: This would make the man Rahul's great-uncle.

3. Great grandfather: This means the man is two generations above Rahul, matching the logical conclusion.

4. Father's brother: This would make the man Rahul's uncle.