It doesn’t matter if Professor Serfin has compelling evidence! We only care about what Professor Serfin believes—and he could believe that Walters committed plagiarism whether or not he has convincing evidence.

We don’t care what Walters would have done in some hypothetical scenario—we don’t even care if Walters committed plagiarism! We only care if Professor Serfin believes that Walters knowingly presented someone else's ideas without attribution, and (B) says nothing about that.

As shown in the diagram below, “shouldn’t officially determine plagiarism” is a necessary condition of “professor doesn’t believe student knowingly presented someone else’s ideas without attribution.” (C) says Professor Serfin satisfies the sufficient condition, so we can conclude that the necessary condition is true!

Of course Walters doesn’t want a determination of plagiarism against him! We don’t care what Walters thinks. We only care what Professor Serfin believes, because that’s the sufficient condition for “should not make an official plagiarism determination.”

(E) doesn’t address the premise’s sufficient condition, so it doesn’t help us conclude the necessary condition. Futhermore, we don’t care what Professor Serfin will or won’t do—this argument is about what he should or shouldn’t do.