Explanation
The Supreme Court of India is the custodian of the Constitution of India. The Constitution is the supreme law of the land and it establishes the framework for the governance of India. It lays down the fundamental principles and rules that govern the functioning of the government, the rights and duties of citizens, and the relationship between different organs of the government.
The Supreme Court of India is the highest court of the land and is entrusted with the responsibility of interpreting the Constitution and ensuring that its provisions are upheld. The Court has the power of judicial review and can strike down any law or action of the government that is found to be unconstitutional.
In addition to the Supreme Court, there are other institutions that are responsible for upholding the Constitution, such as the President and Parliament. However, the primary responsibility of ensuring that the Constitution is upheld lies with the Supreme Court.
So, the correct answer is option D: Supreme Court of India.
