Pointing to a girl during his reception party, Rahul said, "She is the sister of my mother's husband's only daughter-in-law". How is the girl related to Rahul?

Sister-in-law

The correct answer is Option (3) → Sister-in-law

To solve this blood relation problem, we can break down Rahul's statement piece by piece from the end to the beginning:

1. "My mother's husband": This refers to Rahul's Father.
2. "My mother's husband's only daughter-in-law": This refers to Rahul's Father's only daughter-in-law. Since it is Rahul's reception party (meaning he is the one getting married), this person is Rahul's Wife.
3. "Sister of... [Rahul's Wife]": The girl is the sister of Rahul's wife.

Conclusion

The sister of one's wife is referred to as a Sister-in-law.