Concept:
Blood relation problems are best solved by breaking the statement into smaller segments and moving from the end to the beginning.
Step 1: Decoding the first part of the statement.
"My grandfather’s only son" refers to
Riya's Father.
Now substitute this back into the sentence.
Step 2: Decoding the middle and final part.
"The only daughter of [Riya's Father]" refers to
Riya herself.
(Since she says "only daughter," she is referring to her own identity).
Finally, the statement says: "He is the
son of [Riya]."
Thus, the boy in the photograph is Mrs. Riya's
son.