Answer: A
Correct answer: The Chief Justice of India
If both offices are vacant, the Chief Justice of India discharges the functions of the President, and if that office too is vacant, the senior-most judge of the Supreme Court available. This is not in the Constitution itself but in the President (Discharge of Functions) Act, 1969, passed when Zakir Husain died in office and V. V. Giri, then Vice President, resigned to contest the presidential election, leaving both posts vacant. Justice M. Hidayatullah, then Chief Justice, acted as President for about a month in 1969 - the only occasion on which the provision has been used. The ordinary case is simpler. If the President’s office alone falls vacant, the Vice President acts, and an election must be held within six months. V. V. Giri did so on Husain’s death, and B. D. Jatti on Fakhruddin Ali Ahmed’s in 1977. A vacancy in the Vice President’s office is filled by election as soon as possible, with no acting arrangement, since the Deputy Chairman presides over the Rajya Sabha meanwhile.