Everyone knows that the book can’t achieve for many people what the publisher claims it can. So, it’s not very reasonable for people to accept that claim. This triggers the contrapositive of (A), leading to the author’s conclusion: the claim isn’t unethical.

This tells us when knowingly making a false claim is unethical. But we want to support the conclusion that knowingly making such a claim is not unethical. So (B) can’t help us.

For this to support the conclusion, we’d first need to fail the necessary condition, thus triggering the contrapositive. But the premise is silent on this necessary condition. We don’t know whether anyone suffers hardship.

For (D) to work, the premise would need to suggest that there’s no chance anyone will act as if the publisher’s promise might be true. This would trigger the contrapositive. But the premise tells us what all people know, not how all people act on that knowledge.

This tells us when knowingly making a false claim is unethical. But we want to support the conclusion that knowingly making such a claim is not unethical. So (E) can’t help us.