The correct answer is D. His own discretion.
The Governor of a state is the head of the state and is appointed by the President of India. The Governor has the power to appoint the Chief Minister and other ministers of the state. In case no party enjoys absolute majority in the Legislative Assembly of a state, the Governor has the discretion to invite any party or group of parties to form the government. The Governor can also dissolve the Legislative Assembly and call for fresh elections.
The other options are incorrect. The former Chief Minister is no longer in power and does not have any authority to advise the Governor. The Prime Minister is not directly involved in the affairs of the states. The President of India can only advise the Governor, but the Governor is not bound to follow the President’s advice.