Introducing a girl, a boy said, "She is the daughter of the mother of the…

To decode a nested kinship phrase, resolve it working from the innermost relation outward, replacing each phrase with the single relative it names, until only one relation to the speaker remains. Two rules matter here: the daughter of one's aunt is one's cousin, and the mother of a particular person is simply that person's own mother - so 'the mother of a woman's daughter' is the woman herself, not a different person.

1. Start with the innermost relation: 'my aunt' is the boy's aunt - call her Aunt A.

2. 'The daughter of my aunt' is Aunt A's daughter, which makes her the boy's cousin.

3. 'The mother of the daughter of my aunt' is the mother of that daughter - since Aunt A is the daughter's own mother, this phrase again points to Aunt A.

4. 'The daughter of the mother of the daughter of my aunt' is therefore the daughter of Aunt A once more - the same cousin identified in step 2.

5. So the girl introduced is the boy's cousin.

Check by assigning a name: let Aunt A's only daughter be Priya. The daughter of the aunt is Priya; the mother of Priya is Aunt A; the daughter of Aunt A is Priya again. The chain consistently resolves to Priya, who is the boy's cousin, confirming the result.

Answer: Cousin.