Virendra's father-in-law is his wife's father.
The father-in-law's only daughter is Virendra's wife.
The girl is described as "the youngest daughter of my father-in-law's only daughter".
This means the girl is the youngest daughter of Virendra's wife.
Therefore, the girl is Virendra's daughter.
The question asks how Virendra is related to the girl's mother.
The girl's mother is Virendra's wife.
Therefore, Virendra is the husband of the girl's mother.
The answer is Option 2: Husband.