Pointing to a boy, a woman said, “He is the son of my mother’s only son.” How is the boy related to the woman?

The correct option is Nephew.

To determine the relation between the boy and the woman, let us break down the statement step-by-step from the woman's perspective:

1. "My mother’s only son":
The woman is speaking about her mother. Her mother's son would be the woman's brother. Since the mother has only one son, that son is the woman's brother.

2. "He is the son of [my mother’s only son]":
Substituting "my mother's brother/only son" with "her brother", the statement becomes: "He is the son of my brother."

3. Conclusion:
The son of one's brother is one's nephew. Therefore, the boy is the woman's nephew.