Verbal Reasoning - Blood Relation Test - Discussion

The answer for the question may be (A) (C) (D). Because the question only mentions that the grandfather only had one son but not only one child. From this we may come to a conclusion that Lata may also be the daughter of the mother and the man in the photograph may be her cousin. So here we can say that the data provided is not sufficient enough to come to a conclusion.

My doubt is if it mentioned the only son of a grand Father. Does it means grandfather has an only son (one child) no daughter nor other sons?

The question mentioned that his grandfather has only one child is that his father, the second part said that the boy in the photograph is his father's son. So it's clear that his father has two child lata himself/herself and the boy who is in the photograph. So the boy is a brother to Lata both are siblings.