Let "my daughter's father" be analyzed first.
Rahul is speaking about his daughter, so "my daughter's father" = Rahul
Now "the father of my daughter's father" can be determined.
Since "my daughter's father" = Rahul, then "the father of my daughter's father" = the father of Rahul = Rahul's father
The statement says "His only brother is the father of my daughter's father."
This means: The gentleman's only brother = Rahul's father
If the gentleman's brother is Rahul's father, then the gentleman is the brother of Rahul's father.
The brother of Rahul's father is Rahul's uncle.
Therefore, the gentleman is Rahul's Uncle.
The answer is Option 4: Uncle.