Which doctrine may courts use to estop,or prevent,an offeror from revoking an offer based on a party's taking steps in reasonable reliance on an offer?

Question

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If the offeree had reasonably relied on the offeror's promise to hold the offer open and had taken action in reliance on the offer, the courts may use the doctrine of promissory estoppel to estop, or prevent, the offeror from revoking his offer.

Which Doctrine May Courts Use to Estop,or Prevent,an Offeror from Revoking