Final answer:
The completeness assertion for billing is supported by ensuring that all shipments have been billed. This shows that all existent transactions have been recorded, meeting the criteria for completeness in financial reporting.
Step-by-step explanation:
The assertion of completeness in the context of audit and accounting refers to ensuring that all transactions that should have been recorded have been recorded in the correct accounting period. When applied to the billing function of a company, this assertion addresses whether all goods shipped or services provided have been appropriately invoiced to customers.
In the options provided, A) Making sure that all shipments have been billed is the procedure that provides evidence for the completeness assertion. If every shipment is matched with an invoice, it implies that all transactions for goods shipped have been included in the billing process, and thus, are likely to be completely recorded in the financial statements.
Other options, such as ensuring no duplicate billing or that the billing is correct in amounts or to the right customer, focus on different assertions like accuracy, occurrence, and rights and obligations. These are important assertions, but they do not directly provide evidence for the completeness assertion.