Choice "C" is correct. Tracing from a sample of shipping documents to matching sales invoices would provide support for the completeness assertion for billing.
Choice "a" is incorrect. Scanning the sales journal for sequential and unusual entries tests the completeness and existence assertions for sales, but would not provide assurance that shipped goods were properly billed.
Choice "d" is incorrect. Comparing the accounts receivable ledger to the daily sales summary helps ensure that all sales were recorded as receivables and all receivables were recorded as sales, but it does not provide assurance that shipped goods were properly billed.
Choice "b" is incorrect. Inspecting the consecutive numbering of unused sales invoices might identify fictitious sales, but it would not ensure that goods that have been shipped were properly billed.
