Option 1: Justice Mehar Chand Mahajan - He did not have the opportunity to act as President of India during his tenure as Chief Justice of India.

Option 2: Justice P.B. Gajendra-gadkar - He also did not have the opportunity to act as President of India during his tenure as Chief Justice of India.

Option 3: Justice M. Hidayatulla - He is the correct answer. Justice M. Hidayatulla had the opportunity to act as President of India. He served as the Acting President of India from July 20, 1969, to August 24, 1969.

Option 4: Justice P. N. Bhagawati - He also did not have the opportunity to act as President of India during his tenure as Chief Justice of India.

So, the correct option is 3. Justice M. Hidayatulla.