Who/Which of the following is the custodian of the Constitution of India?

1. A

   The President of India

   The President is the constitutional head of the Union and is sworn to preserve, protect and defend the Constitution, but the institution described as custodian through judicial review is the Supreme Court. This option is not the key.

2. B

   The Prime Minister of India

   The Prime Minister heads the Council of Ministers and is the real executive, not the custodian of the Constitution. This option is therefore wrong.

3. C

   The Lok Sabha Secretariat

   The Lok Sabha Secretariat services the House; it has no power to interpret or enforce the Constitution as a court. This option is not the key.

4. D

   The Supreme Court of India

   The Supreme Court of India is the final interpreter of the Constitution and exercises judicial review. In that sense it is the custodian of the Constitution. This option is the official key.

Summary. Official key is (d) The Supreme Court of India. Custody here means final interpretation and enforcement through judicial review, including the basic-structure doctrine. The President takes an oath to defend the Constitution but does not sit as its judicial guardian. The Prime Minister and the Lok Sabha Secretariat are executive and legislative-support offices. Only the Supreme Court fits the custodian role asked.