Which one of the Chief Justices of India has acted as Acting President of India also?

Justice M Hidayatullah, while Chief Justice of India, acted as President for about a month in July and August 1969 under the President (Discharge of Functions) Act 1969, when the offices of both the President and the Vice-President were vacant. Justices Subba Rao and Khanna never discharged the functions of the President, so "Justice Hidayatullah" is the answer.