Neeraj says: "Her sister's father is the only son of my grandmother."
Let's break down this statement:
Rita's sister's father = Rita's father (since they share the same father)
The only son of Neeraj's grandmother = Neeraj's father
From Neeraj's statement:
Rita's father = The only son of Neeraj's grandmother
Rita's father = Neeraj's father
Since Rita and Neeraj have the same father, they are siblings.
Therefore, Neeraj is Rita's brother.
The answer is Option 1: Brother.