Final answer:
The offer to purchase contract becomes a binding contract between the seller and purchaser when the seller has accepted the offer to purchase.
Step-by-step explanation:
The correct answer is a. At the moment the seller has accepted the offer to purchase.
In contract law, an offer becomes a binding contract at the moment the seller accepts the offer. This means that once the seller agrees to the terms and conditions of the offer, a contractual agreement is formed between the seller and the purchaser.
For example, if a buyer makes an offer to purchase a house and the seller accepts the offer, a binding contract is created between the two parties. Both the buyer and the seller are now legally obligated to fulfill the terms of the contract.