Which one is correct?

9. What is the best definition of a contract? a. A voluntary agreement by two or more parties that to perform or refrain from performing some act in the now or in the future which is enforceable in a court of law in the event one of the parties does not comply with a provision of the contract. b. An agreement by only two parties that may or may not be enforceable in a court of law. c. An agreement by only two parties that may be enforceable in court d. An understanding between two or more parties that they may have an agreement