The interviewer himself gives a hint.
He says it’s the last question of the interview.
So the candidate can just has to put his finger confidently at any point of the table which he thinks to be as the center point and tell the interviewer that this is the exact center point.
If the interviewer asks, “How do you know this is the exact center?”
He can reply saying that the last question was the previous question and thus he is not liable to answer further.