The correct answer is (a) The President.
The President of India is the head of state and has the power to withhold assent to a Bill, after it has been passed by the two Houses of Parliament. The President can also return a Bill to the Houses for reconsideration, with or without recommendations. If the Bill is passed again by both Houses, with or without amendments, the President has to give his assent.
The Prime Minister is the head of government and is responsible to the Lok Sabha. The Speaker of the Lok Sabha is the presiding officer of the Lok Sabha and the Chairman of the Rajya Sabha is the presiding officer of the Rajya Sabha. Neither the Prime Minister, the Speaker of the Lok Sabha, nor the Chairman of the Rajya Sabha has the power to withhold assent to a Bill.