Oscar mentions that he has been accused of plagiarizing Ethel Myers's work in his article. He states that this accusation is unwarranted — the main point of his argument. He concedes that he did use passages from Myers's book without attributing them to her, but supports his main point on the basis that Myers privately gave him permission to do this.

Millie counters Oscar's argument by claiming that Myers cannot give Oscar permission to plagiarize. This is because plagiarism affects not only the author by violating their rights, but also misleads readers, making it a lie — and, Millie argues, a lie is still a lie even if another person agrees to it.

Based on the question stem, we're just interested in Oscar's argument. We want to find a principle that justifies Oscar's argument. For a Sufficient Assumption question like this one, a great way to pre-phrase the assumption that is needed is just to look at the premise → conclusion structure.

Oscar's only premise is that Myers gave him permission to use passages from her book without attribution. From this premise, he concludes that there is no warrant to accuse him of plagiarism: i.e., he did not plagiarize. In other words, his argument goes:

permission from author → /plagiarize

So the principle we're looking for will at minimum cover this premise → conclusion gap. It might say something like: if an author grants permission to use passages of their work without attribution, then someone can do so legitimately without plagiarizing.

Note that even though we're not interested in Millie's argument, a principle like the one above would already respond to her argument. If an author granting permission is sufficient to make something "not plagiarism," then Oscar is correct and Millie is wrong, regardless of the effect of that behavior on readers (which is what Millie's argument focuses on).