Attributes sampling, used primarily for tests of controls, looks for the appearance of a specific characteristic (or attribute) in a sample. For example, the auditor may perform a test to determine if every requisition in a sample has supervisor approval.
Variables sampling, also called quantitative sampling, is used for substantive testing. It assists the auditor with determining the reasonableness of a financial statement item. For example, the auditor may use variables sampling to test if the accounts receivable balance on the financial statements is recorded at the correct amount.
According to these definitions, the auditor would use variables sampling, rather than attributes sampling, to estimate whether the dollar amount of inventory is reasonable. The other choices deal with the proper (or improper) recording of items, versus a dollar value.