Choice "B" is correct. Formation of a contract requires acceptance of an offer before the offer is terminated. Jackson sent Sands an offer and then sent a revocation of the offer, and Sands accepted after receiving the revocation. However, the revocation was invalid because the offer was a merchant's firm offer―an offer made by a merchant in writing giving assurances that it will be kept open. Such offers are irrevocable for the time stated, or if no time is stated for a reasonable time, but in no event longer than three months. Thus, Jackson's offer was irrevocable for a reasonable time. It was accepted within a reasonable time (a few days after the offer was received). Moreover, the contract was accepted when Sands mailed the acceptance because of the mailbox rule―an acceptance is effective upon being dispatched.Choice "d" is incorrect. Offers need not state an expiration date to be valid. Absent an expiration date, offers are open for a reasonable time.Choice "a" is incorrect. As explained above, the revocation was ineffective because the offer was a merchant's firm offer and could not be revoked for a reasonable time.Choice "c" is incorrect. Unless the offer provides otherwise, it is effective upon dispatch rather than receipt.