Not necessary, because even if some true literary works can have a harmful effect on society, that doesn’t change the fact that ads harm society. A literature department can still have responsibility to help people avoid the harm of ads, regardless of what’s true about true literary works.

Not necessary, because the author never concludes that a literature department cannot also cover true literary works, too. The conclusion is that the department should cover ads, but that doesn’t imply that true literary works should not be covered.

Not necessary, because even if students who take courses in the literature department DO get other skills from those courses, that doesn’t change the fact that they can improve their skills at analyzing texts, and those improved skills can still help them interpret ads. The author brought up the improved textual skills imparted by literature courses as a reason they can help reduce the harm of ads; not to suggest that literature courses aren’t helpful in any other way.

Not necessary, because even if forms of ads that are entirely visual DO have a harmful effect on society, that doesn’t change that fact that we know from Wigmore’s statements that ads have a harmful effect on society, and that this is due to people’s inability to detect the ads’ real messages. Literature courses can still help people analyze whatever aspect of these ads is textual.

Necessary, because if this were not true — if the literature department’s responsibility IS limited to teaching students how to analyze true literary works — then Wigmore cannot conclude that the literature department’s responsibility includes the study of ads, without knowing whether ads are true literary works.