In Relation puzzle type, mutual blood relations or other information of more than two persons are mentioned and information about any two is mentioned.
Rahul`s mother is the only daughter of Monika`s father. How is Monika`s husband related to Rahul ?
Clearly, the only daughter of Monika's father is Monika herself.
So, Rahul's mother is Monika.
Thus, Monika's husband is the father of Rahul.