Fill in the blank in the following sentence with the most appropriate…

The correct answer is Option (4) → any other

Comparative with "any other":

• When one member of a group is compared with the rest of the same group using a comparative (more polluted than), the pattern is comparative + than + any other + singular noun.
• "Other" leaves the thing itself out of the comparison, so it is not compared with itself.
• Deciding clue: "more polluted than ... river": a comparative with a singular noun of the same group.
• Two-step check: 1. number: singular "river" → any other / other; 2. a singular countable noun needs a determiner → any other.
• This sentence: the Yamuna is one of the rivers of the country, so it is "more polluted than any other river".
• Complete sentence: Yamuna is more polluted than any other river in this country.
  Example: Mumbai is bigger than any other city in Maharashtra.

Why the Other Options are Incorrect?

Option 1: other — Incorrect. A singular countable noun such as "river" needs a determiner before "other" (any, the, every); "than other river" is not correct.
Example: Do you have any other idea?

Option 2: all other — Incorrect. "All other" takes a plural noun (all other rivers); "river" is singular.
Example: All other students had left.

Option 3: all of the — Incorrect. "All of the" also needs a plural noun (all of the rivers), and without "other" it would include the Yamuna itself in the comparison.
Example: All of the rivers were in flood.

Contrast: "any other" takes a singular noun (any other river); "all other" takes a plural noun (all other rivers).