Final answer:
An auditor may compare the quantities on a sales invoice with the quantities on underlying shipping documents to verify sales transactions.
Step-by-step explanation:
An auditor might perform a vouching procedure whereby the auditor compares the quantities on a sales invoice to the quantities on underlying shipping documents. This is done to verify the accuracy and completeness of the sales transactions. By checking if the quantities match, the auditor can ensure that the goods that were sold actually shipped.
In option 1, comparing the quantities on a sales invoice to the quantities on underlying shipping documents is a correct vouching procedure.