The correct answer is the Chief Justice of India.
Article 60 of the Indian Constitution says that the President must take an oath before entering office. This oath is administered by the Chief Justice of India. If the Chief Justice is not available, the senior most judge of the Supreme Court present at that time administers the oath instead.
In the oath, the President promises to protect, preserve, and defend the Constitution and the law, and to work for the welfare of the people of India.