When does the legal concept of promissory estoppel apply

A. when the parties have created a valid contract

B. when an agreement lacks all of the elements to create a enforceable contract

C. when a party, in reliance on the promise of another \, acts to thier detriment and the only way to avoid an injustice is to enforce the promise

D. when the parties have begun the negotiations for a contract but have not completed those negotiations