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D is corrent because when client-prepared statements of account show details of account balances, it is likely that customers will be able to reply regardless of whether they maintain their payable records by invoice or month-end balance.
A is incorrect because the confirmation request is about outstanding balances, not invoices already acknowledged by the client to have been paid by the customers.
B is incorrect because copies of shipping documents would be most likely to be appropriate when issues as to the actual shipment of goods arise.
C is incorrect because while purchase orders may help the customers to determine purchases for which they owe at year-end, they may well not have exact amounts and are not as helpful as details of the account balance.