Pointing towards a boy, Veena said "He is the son of only son my grandfather." How is that boy related to Veena?

The correct answer is C. Cousin.

Veena's grandfather has only one son, who is Veena's father. The boy is the son of Veena's father, so he is Veena's cousin.

Option A is incorrect because the boy is not Veena's uncle. Veena's uncle is the brother of her father or mother.

Option B is incorrect because the boy is not Veena's brother. Veena's brother is the son of her parents.

Option D is incorrect because the question provides enough information to determine the relationship between Veena and the boy.

Option E is incorrect because the boy is related to Veena.