Introducing a boy, a girl says, "He is the son of my mother's only son." Who is the boy to the girl?

The correct option is Nephew.

Let's break down the relationship statement step-by-step from the girl's perspective:

1. "My mother's only son": The girl is speaking about her mother. Her mother's son would be the girl's brother. Since it says "only son", her mother has only one son, which means he is the girl's brother.
2. "He is the son of [my mother's only son]": Replacing "my mother's only son" with "my brother", the statement becomes: "He is the son of my brother."
3. The son of one's brother is one's nephew.

Therefore, the boy is the girl's nephew.