Concept: M.C. Mehta cases are very important in Indian environmental law. They expanded environmental jurisprudence through Public Interest Litigation.
Step 1: Understand the issue.
The question deals with pollution of river Ganga due to failure of municipalities and industries to control pollution.
\[
\text{Municipal inaction} \Rightarrow \text{Ganga pollution}
\]
Step 2: Identify the case.
The Ganga pollution issue was highlighted in M.C. Mehta v. Union of India, 1988.
\[
\text{Ganga pollution case} \Rightarrow \text{M.C. Mehta v. Union of India}
\]
Step 3: Choose the correct option.
Among the given options, M.C. Mehta v. Union of India (1988) is correct.