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.