According to the context of the sentence, the word that best fits is "high" as it is speaking of something dangerous.Also, "high" is the best alternative to modify the noun "risk" taking into account the meaning as a whole.
in this case, if somenthing is dangerous, it cannot be "Low" or "medium" risk so options C and D are not suitable.
As regards the adjective "negative" it does not "collocate" with the noun "risk".
Fact based claims are statements which describes how things were in the past, how they are in the present or how they will be. In this example, the increase in water pollution levels is explained to the growing amount of industrial waste being dumped into waterways
