Thanks for the question! So in this stimulus, Professor Riley is saying that a university president’s speech was inflammatory and thus, inappropriate. The author of the stimulus, however, is saying that we shouldn’t conclude that the speech was actually inflammatory, and thus, that the speech actually wasn’t inappropriate. Prephrasing a little, one problem we might see with the author’s argument is that even if the speech wasn’t inflammatory, it might be inappropriate for other reasons. There might be other conditions that make the speech inappropriate that were met. This is essentially what (A) tells us.
(B), however, is incorrect because the author isn’t arguing that the speech is inflammatory; the author is arguing that the speech wasn’t inflammatory (absent other independent grounds). Thus, whether or not a speech that is inflammatory is appropriate for audiences is irrelevant.
Hope this helps! Feel free to ask any further questions that you might have.