Pointing to a photograph, a woman said to a man, "She is the only child of the sister of the only son of your paternal grandfather." How is the man related to the person in the picture?

Pointing to a photograph, a woman said to a man, “She is the only child of the sister of the only son of your paternal grandfather”.

Family Chart:

So, the man is Mother's Brother's Son of the person in the picture. 

Hence, the correct answer is (b).