Introducing Reena, Monika said, “She is the only daughter of my father’s only daughter.” How is Monika related to Reena?

The correct answer is A. Aunt.

Monika’s father has only one daughter, who is Reena’s mother. This makes Monika Reena’s aunt.

Option B is incorrect because Reena is Monika’s niece, not her daughter.

Option C is incorrect because Reena and Monika are not cousins. Cousins are children of two siblings. In this case, Monika’s father is Reena’s mother’s father, so they are not related in this way.

Option D is incorrect because there is enough information to determine the relationship between Monika and Reena.

Option E is incorrect because Monika is related to Reena.