Final answer:
A document that signals a customer's request for merchandise is a sales order. This is a purchase order issued by the buyer, different from a sales invoice or vendor invoice, which are for payment requests after the sale.
Step-by-step explanation:
A document that indicates a request for merchandise by a customer is known as a sales order. When a customer decides to purchase goods or services, they issue a sales order to the vendor detailing the specifics of the order, such as the types and quantities of products, prices, and delivery instructions. Unlike a sales invoice or a vendor invoice which are issued by the seller to request payment for goods or services provided, a sales order is effectively an order for purchase issued by the buyer. Therefore, the correct answer to the question is D) sales order.