Choice "D" is correct. In general, an audit procedure can be restated as the question to be answered. In this case, tracing a sample of purchase orders and related receiving reports to the purchases journal and the cash disbursements journal seeks to answer the question, "Were all purchases properly recorded?" (the completeness assertion).
Choice "b" is incorrect. Tracing a sample of source documents to summary records would not identify unusually large transactions requiring additional attention.
Choice "c" is incorrect. In order to verify that cash disbursements were for goods actually received, a sample of cash disbursements should be vouched back to receiving reports.
Choice "a" is incorrect. In order to verify that cash disbursements were for goods actually ordered, a sample of cash disbursements should be vouched back to purchase orders.