The correct answer is B. Nephew.
The girl’s uncle is the son of her father’s mother. The boy is the son of the daughter of the girl’s uncle. This means that the boy is the grandson of the girl’s father’s mother, and therefore the girl’s nephew.
Option A is incorrect because the boy is not the girl’s brother. The boy is the son of the daughter of the girl’s uncle, while the girl’s brother is the son of the girl’s mother and father.
Option C is incorrect because the boy is not the girl’s uncle. The boy is the son of the daughter of the girl’s uncle, while the girl’s uncle is the son of the girl’s father’s mother.
Option D is incorrect because the boy is not the girl’s son-in-law. The boy is the son of the daughter of the girl’s uncle, while the girl’s son-in-law is the husband of the girl’s daughter.