In decipher jumbled up description type of questions, a round-about description is given in the form of certain small relationships and direct relationship between the persons concerned is to be deciphered.
Pointing to a girl in the photograph, Amar said, "Her mother`s brother is the only son of my mother's father." How is the girl`s mother related to Amar ?
Only son of Amar's mother's father -- Amar's maternal uncle.
So, the girl's maternal uncle is Amar's maternal uncle.
Thus, the girl's mother is Amar's aunt.