Final answer:
The auditor's review of evidence following up on errors reported by customers supports the completeness assertion in auditing, ensuring that all transactions and events that should be recorded are indeed included in the financial statements.
Step-by-step explanation:
When an auditor observes that a client mails monthly statements to customers and reviews evidence of follow-up on errors reported by customers, this test primarily supports the completeness assertion. This assertion addresses whether all transactions and accounts that should be recorded are in fact recorded in the financial statements. By checking follow-up actions on customer-reported errors, the auditor ensures that the client's accounting systems are capturing, processing, and reporting all of the necessary and accurate information regarding customer transactions, which relates directly to the completeness aspect.
In auditing terms, completeness is concerned with whether all transactions and events that should have been recorded have been recognized and are included in the financial statements. A company sending out statements and subsequently rectifying any discrepancies reported by customers demonstrates a process to complete and correct the record-keeping as necessary.