Unreachable conclusion. The first principle does not allow us to prove that a contract IS valid. It can only allow us to prove that a contract is NOT valid (or that one party accepted a legitimate offer from another).

Doesn’t give us enough to trigger the first principle. We know that if there wasn’t a legitimate offer, the contract isn’t valid. But we don’t know that there wasn’t a legitimate offer, because we don’t know whether a person in Kenta’s position would reasonably know that Gus’s offer was in jest.

Neither principle allows us to conclude that someone will accept an offer.

Unreachable conclusion. The first principle does not allow us to prove that a contract IS valid. It can only allow us to prove that a contract is NOT valid (or that one party accepted a legitimate offer from another).

(E) establishes that the offer Sal made was not legitimate. Based on the first principle, then, there was no valid contract between Sal and Veronica. (Technically (E) doesn’t address the possibility that Veronica made an offer to Sal, but this is the best answer. No other answer comes close to validly using one of the principles. If we interpret the conclusion in (E) as asserting there’s no valid contract based on Sal’s offer, it’s easier to understand why (E) is correct.)