Though the correct answer has been distinguished and partially explained, I feel that there's room to look a bit deeper. And isn't that what we're all here to do?
Conclusion: Scientists were threatened and trying to discredit theory
Premise: Smith published paper that disagreed with other scientists, those scientists are trying to prove it wrong
Assumption: The scientists don't have any other reason to prove it wrong but to discredit it
Notice that this assumption falls into an "eliminate alternate paths" category. Basically, we're told that something is happening, and then directed towards the reason why. But there is no justification for that reason, so we would need an assumption that eliminates other possible reasons. The correct answer will inevitably take advantage of this.
(A) The author passes over the possibility that Professor Smith had much to gain should Professor Smith’s discovery have found general acceptance.
PROBLEM: What we care about is the intentions of the other scientists, not of Professor Smith himself.
(B) The author fails to mention whether or not Professor Smith knew that the existence of the alleged new effect was incompatible with established scientific theory.
PROBLEM: Once again, the conclusion here relates to the OTHER scientists, not Smith himself. Whether or not Smith knew if his research was faulty or not doesn't affect the intentions of the other scientists.
(C) The author fails to show why the other scientists could not have been presenting evidence in order to establish the truth of the matter.
ANSWER: This establishes that the other scientists could have had some motive other than petty payback.
(D) The author neglects to clarify what his or her relationship to Professor Smith is.
PROBLEM: This is entirely out of scope. We don't need to know where the author comes into things.
(E) The author fails to indicate what, if any, effect the publication of Professor Smith’s paper had on the public’s confidence in the safety of most drinking water.
PROBLEM: Out of scope again. We care about Smith and the other scientists, not the public.
Hope that helps!
Tommy Wallach | Manhattan GMAT Instructor | San Francisco
Manhattan GMAT Discount | Manhattan GMAT Reviews