Answer:
a. Statement
Step-by-step explanation:
-A statement refers to a document that allows you to request a payment to a customer that has a debt.
-A purchase order is document that is issued by customers indicating products or services that they will buy to the seller with the agreed terms.
-Sales receipt is a document issued by the seller as a proof of a purchase made by a buyer.
-Invoice is a document issued by a seller that indicates the quantities and prices of products and services provided.
According to this, to collect a payment you should issue a statement to the customer.