Step 1: Background.
M.C. Mehta is a renowned environmental lawyer in India. He filed several PILs on pollution and environment.
Step 2: Landmark judgments.
- Oleum Gas Leak Case (1986): Established "absolute liability" principle.
- Ganga Pollution case: Directed closure of polluting industries.
- Taj Trapezium case: Protection of Taj Mahal from acid rain.
Step 3: Elimination.
- H.S. Dhillon case: Related to taxation, not environment.
- Maneka Gandhi case: Related to Article 21, not environment.
Step 4: Conclusion.
Thus, the leading case on environment is (A).