Final answer:
In an ER diagram for the AP/CD process, VALID_INVOICES are usually received from the Purchasing department, as this department manages the acquisition of goods and services and forwards invoices for payment.
Step-by-step explanation:
In the context of an ER diagram for the Accounts Payable (AP)/Cash Disbursements (CD) process, VALID_INVOICES are typically received from the Purchasing department. The Purchasing department is responsible for acquiring goods or services for the company, and after receiving them, they send invoices to the Accounts Payable department to be processed for payment. Thus, it is the Purchasing department that sends VALID_INVOICES to Accounts Payable in most organizational structures.