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 out to a photograph, a man tells his friend, "She is the daughter of the only son of my father's wife."
How is the girl in the photgraph related to the man ?
The relations may be analysed as follows :
Father's wife -- Mother;
Mother's only son -- Himself.
So, the girl is man's daughter.
Hence, the answer is (a).