Choice "D" is correct. Under the UCC, a contract involving "goods" may be modified without the need for additional (new) consideration. Here, the subsequent modification is binding. Moreover, no writing was required for the modification because the contract price, as modified, was less than $500.
Choice "b" is incorrect. Mason's status as a nonmerchant is irrelevant and Acme is bound to the June 2 delivery date because it was a valid modification. Under the UCC, a modification sought in good faith is binding even absent consideration, and an oral modification is enforceable if the contract, as modified, is not within the Statute of Frauds (i.e., it is for the sale of goods for less than $500).
Choice "a" is incorrect. The oral agreement to change the delivery date is enforceable here, because a contract modification under the Sales Article is enforceable if the contract, as modified, is not within the Statute (i.e., it is for the sale of goods for less than $500). Moreover, the agreement is binding despite the lack of consideration because, under the Sales Article, a modification sought in good faith is binding despite the lack of consideration.
Choice "c" is incorrect. A firm offer is a written offer by a merchant containing words of firmness (i.e., a promise to keep the offer open). The agreement here is not a firm offer, but rather is a modification, an agreement between the parties to change their contract.