Choice "D" is correct. Proper authorization for sales involves obtaining credit approval before filling the order.
Choice "b" is incorrect. Separation of duties between billing and cash receipts relates to the prevention of theft of cash, not to proper authorization of sales.
Choice "a" is incorrect. The use of an approved price list to determine selling price relates to the accuracy and appropriateness of selling price, not to the proper authorization of the sale.
Choice "c" is incorrect. Sending copies of approved sales orders to shipping, billing, and accounting provides information necessary to ensure that the proper goods are shipped, the invoice is prepared appropriately, and the sale and receivable are properly recorded. However, since it is copies of approved sales orders that are being sent, this procedure comes too late in the process to assure that sales were properly authorized in the first place.