Bill gives his nephew Dale $10,000 and says "I hope you go to college someday." Dale turns 18 and enters the military. Bill is disappointed and tells Dale they had a deal and he wants his money back.  

 must return the money. He has breached a contract.

must return the money, since he has been unjustly enriched.

may return the money, but is under no obligation to do so.

must return the money, even though this contract should have been in writing.