This is a method of reasoning question wherein we are tasked by analyzing the reasoning structure of an the arguments and discern the author's argument strategy.
Professor B:
Premise: American sign language is native to many north americans
Sub conclusion: American sign language is not a foreign language
Conclusion: Students should not be permitted to statisfy the foreign language requirement by taking sign language.
Not a very good argument, professor B presumes that if a language is native to many people in a particular geographical then it cannot be a foreign language.
Professor S:
Premise: By your reasoning people who satisfy their language requirements by taking french or Spanish should not be able to, because those languages are native to many north americans.
Conclusion: However, many students satisfy their foreign language requirements by taking those languages and it would be absurd to prevent them from doing so.
Professor S proceeds by arguing that if we accepted the Professor B's reasoning as true, then it would to a less than ideal/nonsensical situation (people who learn spanish and french to satisfy language requirements being prohibited from doing so).
(A) Bingo. This is exact strategy employed by professor S. He uses his reasoning to show that it will lead to a precarious situation.
(B) CAREFUL! This is a trap answer choice that will be tempting if you don't have a clear sense of the argumentative strategy employed by professor S, or if you did not correctly process what this answer choice is saying. Professor S uses Professor B's reasoning to arrive at a conclusion that contradicts the one drawn by Professor B. The conclusions are the only things that are contradicted here. Professor S makes no attempt to demonstrate that the conclusion(s) contradict the reasoning.
(C) Professor S does not engage in ad hominems nor does he question the Professor B's authority to speak on the topic at hand.
(D) While this is a valid way of arguing, such a method is not employed by Professor S. An example of this would be if Professor B stated as a fact that all Americas are skinny and offered as an explanation for this phenomenon that it was because they are malnourished. Professor S would then have to offer an competing hypothesis, and say for example that americans were skinny because they eat healthy. This doesn't happen at all. In fact Professor B doesn't try to explain any fact or phenomenon nor does Professor S try to introduce an alternative hypothesis for such fact/phenomenon.
(E) Professor S does not agree with his Professor B's conclusion, he uses he uses Professor B's reasoning to arrive at a conclusion that directly contradicts the conclusion drawn by Professor B.