Answer: a. Hal and Sophia would still have a duty to perform on the contract, but could then sue Jack for the cost of the cleaning.
Explanation:
Hal and Sophia are party to a contract with Jack and all parties therefore must fulfill the conditions set out.
If contract terms are broken, the offending party could be sued.
Jack has broken a clause in the contract but that should not influence Sophia and Hal to void the contract. They should go ahead and then sue Jack for breach of contract.