31.
A:
M.C. Mehta v. Kamal Nath & others
B:
M.C. Mehta v. Union of India (Ganga water pollution case)
C:
M.C. Mehta v. Union of India (Replacing diesel vehicles by CNG vehicles)
D:
Rural Litigation & Entitlement Kendra v. State of U.P.
The answer is: A