Choice "D" is correct. Examining vendor invoices for amounts not reported as purchases is a procedure to determine if accounts payable have been understated (completeness assertion). Accounts payable would be understated for the amounts of purchases that were not reported.
Choice "b" is incorrect. Examining reported purchase returns that appear too low means that the transactions have been recorded. The auditor would be more interested in purchase returns that have not been reported.
Choice "a" is incorrect. Searching for customer-returned goods that were not reported as returns would have an impact on accounts receivable and sales returns accounts, not accounts payable.
Choice "c" is incorrect. Reviewing bank transfers recorded as cash received from customers has an impact on accounts receivable, not accounts payable.