Final answer:
The purchase order is the document matched with a vendor invoice to ensure a purchase was authorized, ensuring input validity in the business transaction process.
Step-by-step explanation:
The document that would be matched with the vendor invoice to ensure that the purchase was authorized and to ensure input validity is the purchase order (B). A purchase order is an official and binding document issued by a buyer committing to pay the seller for the supply of specific goods or services to be delivered in the future. The purchase order serves as authorization for the expense and is matched with the vendor invoice and the receiving report to verify that the items billed were ordered and received. Matching these documents helps prevent errors and fraud, thereby ensuring the accuracy and legitimacy of transactions.