This is a strange question. For a GMAT question, it comes across as very formal and strict, almost more like an LSAT question. We at Knewton
believe that the answer to this question must be C. Here's our reasoning:
The statement at the beginning of the argument (1) is contradicted by the statement at the end (2), which is "taken to be true" based on the rest of the argument. The middle of the argument, which is more interesting and distracting, is only there to bolster the "truth" of (2), but the argument is designed to contradict (1):
1) Scientists are sometimes said to assume that something is not the case until there is proof that it is the case.
2) But no scientist could assume without contradiction that a given substance is both safe and not safe: so this characterization of scientists is clearly wrong.
Only choice C addresses this contradiction. The others all include "killer" words that overshoot the mark (capitalized below):
(A) A general statement is argued to be false by showing that it has DELIBERATELY been formulated to MISLEAD. (this argument does not claim that anyone is deliberately trying to mislead anyone else; this is far too extreme)
(B) A statement is argued to be false by showing that taking it to be true leads to IMPLAUSIBLE consequences. (implausible only means "difficult to believe," not "logically impossible." Everything else in this answer fits, but if this is the OA, then the question-writer must be unclear about the meaning of "implausible")
(C) A statement is shown to be false by showing that it directly contradicts a second statement that is taken to be true. (correct)
(D) A general statement is shown to be UNINFORMATIVE by showing that there are AS MANY specific instances in which it is false as there are instances in which it is true. (this choice does not address the conclusion of the argument, which is that a statement is "clearly wrong." Also, the relative number of cases in which the characterization is right or wrong is not mentioned.)
(E) A statement is shown to be UNINFORMATIVE by showing that it supports NO independently testable inferences. (again, not strong enough a word. We have no idea what is meant by "independently testable inferences" here, either, but the argument uses one example, so the word "no" is extreme as well.)
This is definitely a tricky question. An important factor to remember on such questions is that the argument does not have to be logically solid for the "method of reasoning" to be identified. This argument is pretty wacky, but the arguer still points out a direct contradiction to prove a statement "false," so C is the best answer. Watch for killer words!