Final answer:
The auditor would select a sample from the bills of lading file to ensure that each shipped order has a corresponding invoice, helping to assess the effectiveness of internal controls in preventing unbilled shipments. so, option b is the correct answer.
Step-by-step explanation:
To determine whether the system of internal control operated effectively to minimize errors of failure to invoice a shipment, an auditor would likely select a sample of transactions from the bills of lading file. The bills of lading file contains records of shipments that have been sent out, which should align with invoices created for those shipments. By verifying that each bill of lading has a corresponding sales invoice, the auditor can assess if there were any shipments that were not invoiced, indicating a lapse in the internal control mechanism related to billing.
In a typical scenario, selecting a sample from the sales invoice file would involve checking for transactions that have been invoiced, which does not directly address the potential error of failing to generate an invoice. The customer order file would show orders placed, but not necessarily shipped, and the open invoice file shows invoices that have not yet been paid, which are not directly indicative of shipments that may have gone unbilled.