Pointing to a man, a woman said, He is the only son of my mother’s mother .How…

Rule: any child of a person's grandparent, other than that person's own parent, is that person's uncle (if male) or aunt (if female). The reciprocal relation is that this person becomes the niece (if female) or nephew (if male) of that uncle/aunt.

1. ‘My mother’s mother’ is the woman’s maternal grandmother.

2. The man is described as the ‘only son’ of that grandmother — so he is a son of the woman’s maternal grandmother.

3. A son of the maternal grandmother, other than the woman’s own mother, occupies the mother’s-sibling generation — he is the mother’s brother.

4. Applying the rule above: the woman, being female, is the niece of her mother’s brother.

Cross-check without assuming family size: the maternal grandmother’s ONLY son is the man himself — she has no other son. The woman’s mother must be one of the grandmother’s daughters (regardless of how many daughters exist). A daughter and the ONLY son of the same parent are always siblings, so the man is confirmed to be the brother of the woman’s mother, matching the attached family-tree diagram below.

Hence, the woman is related to the man as his Niece.