Choice "C" is correct. Peters' offer was already revoked. To be effective, an acceptance must be received before the offer is terminated. An offer may be terminated by the offeror at any time unless the offeree gave consideration to keep the offer open. Mason gave no consideration to keep the offer here open; thus Peters could revoke. An offer is considered to be revoked if the offeree obtains information from a reliable source that the subject matter of the offer has been sold. Here, Mason received the information of the sale before he attempted to accept. Therefore, his attempted acceptance came too late and was ineffective.
Choice "b" is incorrect. The normal rule for contracts is the mailbox rule - an acceptance is effective upon dispatch rather than upon receipt.
Choice "d" is incorrect. While the normal rule is that an acceptance will be effective upon dispatch, to be effective, an acceptance must also come before the offer is terminated. Here the offer was terminated before the letter was sent because Mason received reliable information that the subject matter of the offer had already been sold.
Choice "a" is incorrect. An offer will be irrevocable only if the offeree gives consideration to keep the offer open (an option contract) or the offer is a merchant's firm offer. Here, Mason did not give consideration to keep the offer open, and firm offers must be in writing and made by a merchant, yet the offer here was oral and nothing indicates that Peters is a lawn mower merchant.