The correct answer is option 4, P. N. Bhagwati. Public Interest Litigation (PIL) was introduced in the Indian Judicial System during his tenure as the Chief Justice of India. Let`s briefly look at the other options to understand why they are not the correct answers.

Option 1, M. Hidayatullah, was the Chief Justice of India from 1968 to 1970. PIL was introduced much later, in the 1980s, so it cannot be attributed to him.

Option 2, A. S. Anand, served as the Chief Justice of India from 1998 to 2001. While he played a significant role in judicial reforms, PIL was already in practice before his term as Chief Justice, so he is not the correct answer.

Option 3, A. M. Ahmadi, served as the Chief Justice of India from 1994 to 1997. Similar to option 2, PIL was already in place before his tenure, so he is not the correct answer.

Therefore, the Chief Justice of India who was in office when PIL was introduced in the Indian Judicial System was P. N. Bhagwati.