By using the symbols in the table given below, we can draw the following family tree:
Given: Sayantan introduces Krish as the only child of his father’s father. The word his indicates that Sayantan is a male. Sayantan’s father’s father means Sayantan’s grandfather. The grandfather of Sayantan has only one child who is Krish, this means Krish is the father of Sayantan
According to the tree diagram, Krish is the father of Sayantan. Hence, the correct answer is “father”.