Choice "A" is correct.  Attribute sampling is used by the auditor to test whether the controls put in place by the client operate effectively. By inspecting purchase orders, the auditor can determine whether adequate approvals are in place (documented) to demonstrate that purchases are properly supervised. Choice "c" is incorrect.  Sampling is not used to make an independent estimate of payroll expense.Choice "d" is incorrect. Determining that all payables are recorded at year end would not use attribute sampling because a specific control (or attribute) put in place by the client is not being tested.  The completeness of year-end accounts receivable is typically tested using the search for unrecorded liabilities, which is a substantive procedure. Variables sampling and PPS sampling are typically used in substantive testing.Choice "b" is incorrect. The selection of confirmations to verify account balances is part of substantive testing and would use variables sampling or PPS sampling, not attributes sampling.