Which One of the Following Contracts Is an Example of an Illegal

Which one of the following contracts is an example of an illegal contract, not binding on the parties?

A) A contract between camera dealers and distributors whereby they agree to sell a specific model of camera
B) A contract whereby one person agrees to perform services for another, but payment will be made at the end of the contract
C) A contract whereby one person agrees to buy a car from another person
D) A contract whereby one person agrees to sell a desk to another
E) The act of paying for sexual favours (prostitution)