In case of death of both the President and Vice President of India, who shall act as the President of…

The correct choice is: “Chief Justice of India”

Under the President (Discharge of Functions) Act, 1969, made under Article 70, if both the offices of President and Vice-President are vacant, the Chief Justice of India discharges the functions of the President. If the Chief Justice is also unavailable, the senior-most available Supreme Court judge does so.

This happened in 1969, when Chief Justice M. Hidayatullah acted as President after President Zakir Husain died and Vice-President V. V. Giri resigned to contest the presidential election.