Choice "C" is correct. Shipping documents provide evidence that a sale occurred, and tracing from shipping documents to sales invoices might identify instances in which a sale occurred but the associated invoice was not prepared (i.e., if no related invoice can be found).
Choice "d" is incorrect. Selecting a sample of transactions from the cash receipts file is not an appropriate means of identifying sales for which no invoice was prepared, since it will only include sales for which payment has been received. If an invoice was not prepared and mailed, it is unlikely that the customer will have made such a payment.
Choice "b" is incorrect. Selecting a sample of transactions from the customer order might result in items being selected which are not valid sales, for example, if the order has not yet been filled.
Choice "a" is incorrect. Selecting a sample of transactions from the sales invoice file will prevent the auditor from ever identifying sales for which no invoice exists, since there would be no invoice to select in such cases.