Answer: C. an implied contract.
Step-by-step explanation:
An Implied Contract is one that arises as a result of the way one or both of the parties involved in the contract acts towards the other.
Unlike an Express Contract, it need not be written down but it does have the same legal weight and strength of a written contract.
The basic principle of this contract is that people should always be treated fairly in business transactions so the need to always pen it down is not necessary.
By walking in and leaving his clothes at the laundry, Bill got into an Implied Contract as it would be unfair for Tom to just clean his clothes with no payment.