Final answer:
A sales associate best assists a customer by offering an alternative product and checking the inventory system to inform the customer when the item will be in stock again.
Step-by-step explanation:
The best assistance a sales associate can provide to a customer looking for an item that is out of stock is to offer an alternative that meets the customer's needs. By suggesting a similar or related product, the sales associate can help satisfy the customer's requirements while also maintaining the opportunity for a sale. Additionally, the sales associate can check the inventory system to determine when the product will be back in stock and inform the customer accordingly, perhaps even offering to place the item on hold or arrange for notification upon its arrival.