Introducing a person, X says, "His father is the only son-in-law of my father." How is X related to the person?

The correct option is Mother.

Let us break down the logical relationship step-by-step from the perspective of person X:

1. Identify "my father": This refers to X's father.

2. Identify "the only son-in-law of my father": The son-in-law of X's father would be the husband of X's father's daughter. Since X is introducing the person, the only son-in-law of X's father must be X's husband (and X is female in this context).

3. Now evaluate the full statement: "His father is the only son-in-law of my father."

This means: The person's father is X's husband.

4. Since X's husband is the father of the person, X must be the mother of that person.

Therefore, X is related to the person as Mother.