Under what condition is an offer irrevocable?

An offer is considered irrevocable when detrimental reliance occurs because this principle stems from the doctrine of promissory estoppel. Detrimental reliance happens when the offeree takes significant actions based on the belief that the offer will be honored, leading them to incur costs or change their position in reliance on that promise. If the offeror then decides to back out, it could result in injustice to the offeree, who relied on the offer to their detriment.

This situation creates an ethical obligation for the offeror not to revoke the offer after the offeree has begun to rely on it. The courts may enforce the offer even if it was originally revocable, thereby creating an irrevocable situation due to the actions taken by the offeree based on the offer. This protects the offeree and ensures fairness in business dealings.

The other options do not lead to an irrevocable offer under typical legal principles. A mere acceptance does not have an effect on the offer's status of revocability; the illegality of the subject matter usually revokes the offer rather than making it irrevocable; and a counteroffer typically serves to reject the original offer, rather than sealing its irrevocability.