Correct Answer: (d) Chief Justice of the Supreme Court
Solution:Office can be vacant in the following ways:
1. When the President of India completes his term of five years in the office
2. If the President resigns by putting forward his resignation to the Vice-President of India
3. If Lok Sabha/Rajya Sabha initiates an impeachment charge and they stand valid, he is removed
4. if he dies in the office
5. If the Supreme Court declares his election invalid
Note: Vice-President discharges the duties as President; if the latter's office falls vacant in the circumstances mentioned above, except by the expiry of the term. As per the President's Act 1969; if the Vice-President office is vacant too, Chief Justice of India (CJI) (or in his absence), Supreme Court's senior-most judge, discharge the functions of the President (till new President is elected.)