Final answer:
Joan's past coaching cannot be considered as present or future consideration for Bobby's promise of $500,000 upon signing a professional contract, making the promise unenforceable on these grounds.
Step-by-step explanation:
The question presented involves the concept of consideration within contract law. When Bobby promises to give Joan $500,000 for teaching him the jump shot after he signs a professional contract, this promise may lack consideration, since Joan's act of teaching Bobby occurred in the past. In contract law, for a promise to be enforceable, there typically needs to be present or future consideration - something of value exchanged between the parties at the time of the agreement or promised in the future. As Joan's coaching has already been provided, it cannot serve as consideration for Bobby's new promise. Therefore, option 4 is correct: Joan's work with Bobby in the past is not consideration to enforce Bobby's promise.