When is an offer irrevocable?

An offer becomes irrevocable when performance begins on the offer because this principle relates to unilateral contracts. In a unilateral contract, one party (the offeror) makes a promise in exchange for an act by the offeree. Once the offeree begins the act requested by the offeror, the offer is considered accepted and thus cannot be revoked. This provides the offeree with the assurance that they can complete the act without the risk that the offer will be taken away mid-performance.

The other choices reflect different scenarios where an offer may or may not remain open. Acceptance, while it does bind the offeror and offeree to the terms of the offer, does not address the issue of irrevocability during the performance of the act. A notice of revocation from the offeror would typically invalidate the offer only if it occurs before acceptance or performance has begun. Lastly, a specified period in the offer relates to how long the offer remains open for acceptance but doesn't indicate irrevocability upon the initiation of performance like the correct choice does. Thus, the initiation of performance is the key factor in making an offer irrevocable.