Final answer:
Aaron and Donald's written agreement to supply raw materials in exchange for set fees is an express contract, which is explicitly stated in writing or orally, and is distinct from implied, unilateral, or informal contracts.
Step-by-step explanation:
- Aaron and Donald have entered into an express contract.
- An express contract is a mutually agreed upon arrangement where the terms are declared either orally or in writing at the time of its formation.
- This contrasts with an implied contract, where the arrangement is inferred from the actions or conduct of the parties, or a unilateral contract, where only one party makes an enforceable promise.
- Finally, while the term 'informal contract' could apply to any contract not formalized through a specific legal procedure or requirement (such as a handwritten agreement between friends), an express contract like the one described can be both written and formal.