Step 1: Understand what the question asks.
The question wants to know who first used the actual words "Public Interest Litigation". This is different from asking who developed or applied it in court.
Step 2: Know what PIL means.
PIL lets any public-spirited person go to court for poor or weak people whose rights are hurt. It widened the rule of who can file a case.
Step 3: Find who coined the term.
The phrase "Public Interest Litigation" is credited to Professor Upendra Baxi, a famous legal scholar. He wrote and taught a lot about the courts helping social justice.
Step 4: Check the judges in the options.
Justice P.N. Bhagwati and Justice V.R. Krishna Iyer made PIL strong through their judgments. But they applied it; they did not coin the words.
Step 5: Rule out the wrong options.
Prof. Abram Chayes wrote on public law in the USA, not the Indian PIL term. So options A, B and D do not fit.
Step 6: Pick the correct answer.
The person who first used the term is Prof. Upendra Baxi.
\[ \boxed{\text{Prof. Upendra Baxi}} \]