Choice "A" is correct. Under UCC 2-206, an offer that does not specify the means of acceptance may be accepted by any means reasonable under the circumstances; thus the attempt to accept by mail or fax was a proper means. Generally under the mailbox rule, an acceptance will be effective on dispatch (including sending a fax) unless the offer specifies that acceptance will be effective only upon receipt. Here, the offer merely says that it will remain open for 10 days; it does not require receipt of the acceptance within 10 days. Thus, the mailbox rule applies and both the letter and the fax are valid acceptances.