The correct answer is D. Cousin.
Nikhil said, “I am the only son of her mother’s son.” This means that Nikhil’s father is Rita’s mother’s son. This makes Rita Nikhil’s first cousin.
Aunt: An aunt is the sister of a person’s mother or father, or the wife of a person’s uncle.
Niece: A niece is the daughter of a person’s brother or sister.
Mother: A mother is a woman who has given birth to a child or adopted a child.
Cousin: A cousin is a person who shares one or both parents with another person.