The correct answer is (a) The Vice-President.
The Vice-President is the ex-officio Chairman of the Rajya Sabha. This means that the Vice-President is the Chairman of the Rajya Sabha by virtue of his/her office. The Vice-President is not elected by the members of the Rajya Sabha, but is elected by an electoral college consisting of members of both Houses of Parliament and the Legislative Assemblies of the States.
The Vice-President is the second highest constitutional office in India, after the President. The Vice-President acts as the President when the President is unable to discharge his/her duties due to illness or absence from India. The Vice-President also has the power to nominate 12 members to the Rajya Sabha.
The other options are incorrect. The President is the head of state of India and is elected by an electoral college consisting of members of both Houses of Parliament and the Legislative Assemblies of the States. The Prime Minister is the head of government of India and is appointed by the President. The Speaker is the presiding officer of the Lok Sabha.