Choice "D" is correct. Shipping documents provide evidence that a sale occurred, and therefore selecting from a population of shipping documents allows the auditor to test whether corresponding invoices exist for each sale.
Choices "a" and "b" are incorrect. The existence of customer orders and sales orders does not necessarily imply that a sale occurred. For example, the particular item requested may be out of stock. In such cases, there would properly be no sales invoice.
Choice "c" is incorrect. Since the weakness the auditor is concerned about involves missing sales invoices, selecting from a sample of existing sales invoices would not identify this problem.