Answer:
The correct answer would be, Yes a valid contract has been formed between them on July 15th when the letter of acceptance was sent.
Explanation:
Jacks makes an offer to sell his restaurant on July 10th, and on July 15th, Hall and Sophie mailed him the letter of acceptance. But on the very next day they called Jack and told him that now they don't want to purchase the restaurant. But because they have already signed the contract on 15th of July and it was a legal document, not a verbal one, so it means that both parties are now in a valid contract, as Hal and Sophie sent Jack the contract of acceptance after signing it.
