The correct answer is (d) President.
The President of India is the authority to specify as to which castes shall be deemed to be Scheduled Castes. This is done under Article 341 of the Constitution of India, which states that the President may, with respect to any State or Union territory, by public notification, specify the castes, races or tribes which shall be deemed to be Scheduled Castes for the purposes of the Constitution.
The Commission for Scheduled Castes and Tribes is a statutory body established by the Government of India to protect the interests of Scheduled Castes and Scheduled Tribes. The Commission has no role in specifying which castes shall be deemed to be Scheduled Castes.
The Prime Minister is the head of the Government of India. He/She is appointed by the President of India and is responsible to the Parliament of India. The Prime Minister has no role in specifying which castes shall be deemed to be Scheduled Castes.
The Governor of a State is the head of the State Government. He/She is appointed by the President of India and is responsible to the State Legislature. The Governor has no role in specifying which castes shall be deemed to be Scheduled Castes.