Answer:
a. the subject matter can be either legal or illegal, as long as there was an agreement
Step-by-step explanation:
A contract is not enforceable if the subject of the contract is illegal, Thus any contract with an illegal/unenforceable subject is not legal, Example is the agreement between a drug dealer and a supplier, such agreement has an illegal subject and is thus unenforceable.
The subject matter must be legal, one party makes an offer and the other party accepts an offer.