Final answer:
Ed and Sheila have entered into a valid unilateral contract.
Step-by-step explanation:
Ed and Sheila have entered into a valid unilateral contract.
In this scenario, Ed has agreed to fix Sheila's air conditioner by the end of the day in exchange for a payment of $500. Sheila's promise to pay Ed serves as consideration, which is an essential element for a valid contract. A unilateral contract is formed when one party makes a promise in exchange for the other party's performance. In this case, Ed's fixing of the air conditioner is the required performance, and once he completes it, Sheila is obligated to pay him the agreed amount. This situation does not fall under a bilateral contract where both parties promise to perform or provide something to each other. Therefore, the correct answer is that Ed and Sheila have entered into a valid unilateral contract.