21.
The answer is: A
Explanation
The correct option is A: President.
According to Article 143 of the Indian Constitution, the President may seek the opinion of the Supreme Court on any question of law or fact of public importance on which he thinks it expedient to obtain such an opinion.
This is known as the advisory jurisdiction of the Supreme Court. The opinion of the Supreme Court is only advisory and not binding on the President. The other options are incorrect because the Governor and the Prime Minister do not have the power to seek the advisory opinion of the Supreme Court under Article 143.