The correct option is A Few
The given sentence needs to be filled with the most suitable determiner. Looking at the sentence, we can easily say that the sentence is a general statement. In case of general statements, we must always use the positive degree of determiner, unless the sentence is about comparison. Thus, the correct option is A - 'Few rivers in India are not polluted.' Option B is incorrect, as 'fewest' is the superlative degree of determiner. We can't use the superlative form until and unless we want to state that the noun is in its highest degree. Option C is incorrect, as 'fewer' is the comparative degree of determiner. We can use the comparative form only when there is a comparison between two subjects. Option D is incorrect, as option A is the answer.