Pointing to a lady, Ajay said, "She is the only daughter of my mother's father. She has two children. Her only son's wife is Reeta."Ajay is the lady's __________.

​Pointing to a lady, Ajay said, "She is the only daughter of my mother's father. She has two children. Her only son's wife is Reeta."

The lady (Ajay's mother) has two children, and one of them is a son, that son must be Ajay himself.
And it is given that "her only son's wife is Reeta", which means Ajay is married to Reeta.

So, Ajay is the son of the lady.

Thus, the correct option is: (a) Son.