Pointing to a boy in the photograph Reena said, “He is the only son of the only child of my grandfather.” How Reena is related to that boy?

The correct answer is C. Aunt.

Reena’s grandfather has only one child, who is Reena’s parent. The boy in the photograph is the only son of Reena’s parent. This means that the boy is Reena’s nephew. Reena’s nephew’s mother is Reena’s sibling. This means that Reena is the boy’s aunt.

Option A is incorrect because Reena is not the boy’s mother. Option B is incorrect because Reena is not the boy’s sister. Option D is incorrect because Reena is related to the boy.