Final answer:
The appropriate unit for testing in an A/R confirmation sample is individual customer balances, which enables the auditor to confirm the recorded amounts against customer records directly.
Step-by-step explanation:
The appropriate unit to be tested for an accounts receivable (A/R) confirmation sample would be individual customer balances. This is because the A/R confirmation process involves sending out direct requests to a company's customers, asking them to confirm the amount they owe to the company. Testing individual balances allows the auditor to corroborate the amounts stated in the financial records with the balances as per the customers' records. Total accounts receivable balance is not a unit but a culmination of all individual balances. Sales revenue and inventory turnover ratio are irrelevant to confirmations, as they pertain to income statement and efficiency metrics, respectively.