We’re looking for the claim that if most of the dairies in the central valley DON’T meet federal standards, it’s likely that the water WILL be polluted. (A), however, tells us what happens if most of the dairies DO meet federal standards. This doesn’t establish what happens if most of the dairies DON’T meet federal standards.

Even if (B) could establish that we can’t keep “all” of the drinking water clean, this doesn’t guarantee that “most” of the drinking water is likely to be polluted. For example, perhaps only 1% of the water is unclean. This wouldn’t be a situation in which most (over half) of the drinking water is polluted.

(C) reverses something that would be correct. “Only if” introduces a necessary condition. But we want to know that failure to meet federal standards is sufficient to make most of the drinking water likely to be polluted.

We know from the premises that most large dairies in the central valley don’t meet federal standards. (D) tells us that if that is the case, most drinking water is likely to be polluted.

We do not know from the premises that “none” of the large dairies in the central valley meet federal standards. So the premises would not interact with the conditional in (E). We still would not know whether most of the drinking water is likely to become polluted.