Looking at the portrait of a man, Mohan said, "His mother is the wife of my father's son and I have no brother and sister." at whose portrait was Mohan looking?

His Son

Before answering this question we must be aware of the method to solve the blood relation questions.

For example : 

It is now clear, how to make charts in Blood relations. Now, shifting to another type, in this we solve the question in the following manner.

The chart for the above question is as follows: