Final answer:
Betty's actions at the grocery store formed an implied-in-fact contract, which is established through the conduct of the parties rather than explicit verbal or written communication.
Step-by-step explanation:
Betty has formed an implied-in-fact contract when she took the apple from the grocery store. An implied-in-fact contract is created by the non-verbal conduct of the parties involved, rather than a written or spoken agreement. In this scenario, Betty's acknowledgment of the clerk and the clerk's nod, coupled with the existing open account and Betty's knowledge that she will be billed later, demonstrates an agreement that is understood by both parties without explicitly stating the terms.