Explanation:

- Option 1: Money: In many transactions, money exchanges hands, but a sale isn't complete without ownership transfer.

- Option 2: Goods: The delivery of goods is important, but an actual sale requires more than just physical transfer.

- Option 3: Interest: Transferring interest might imply a stake or advantage, but it doesn't equate to completing a sale.

- Option 4: Ownership: ? Ownership transfer is key to a completed sale, as it signifies that the buyer now possesses the rights to the goods or property.