Step-by-step Solution:

Correct Answer: B (Father-in-law)

The relationship can be understood by breaking down Ajay's statement:

• "the grandson of my mother" refers to Ajay's own son.
• "the wife of the grandson..." is therefore the wife of Ajay's son.
• The wife of one's son is their daughter-in-law.

Since the girl is Ajay's daughter-in-law, Ajay is the girl's father-in-law.