Which of the Following Offerors Is Free to Revoke His

Which of the following offerors is free to revoke his offer?

A) An offeror that agreed to keep an offer open without consideration.
B) An offeror,who is a merchant,agreed in writing to sell goods and gave assurances that the offer would remain open.
C) An offeror of a unilateral contract where the offeree already started performance.
D) An offeror who promised to keep an offer open without consideration,but later changed his mind and the offeree foreseeably and reasonable relied on the offeror's promise.