Let's break down the relationship described in the statement.
Vivek says: "This boy is the husband of the only granddaughter of my father."
Starting from Vivek's perspective:
"My father" refers to Vivek's father.
"The only granddaughter of my father" refers to the granddaughter of Vivek's father.
A granddaughter of Vivek's father would be the daughter of Vivek's father's child. Since Vivek is a child of his father, the granddaughter would be Vivek's daughter.
Now we can establish the relationship:
The only granddaughter of Vivek's father = Vivek's daughter
"This boy" = The husband of Vivek's daughter
The husband of one's daughter is called a son-in-law.
From the boy's perspective, Vivek would be his father-in-law.
Therefore, Vivek is related to the boy as his Father-in-law.
The answer is Option 3: 3