What factors will a U.S. court likely use to determine whether a written agreement which is not a formal contract can still be enforced based upon promissory estoppel?

A the understanding of such letters in the specific business area and unwritten representations if any;

B whether attorneys were involved;

c whether the creditor was reasonable in relying upon the letter under an implied contract, promissory estoppel or detrimental reliance theory.

D all of the above