Choice "C" is correct. To provide assurance that each voucher is submitted and paid only once, an auditor should verify that each voucher was stamped "paid" by the check signer.
Choice "a" is incorrect. Paid vouchers should be returned to the accounting department for posting of the payment and filing of the documents, but this does not prevent the voucher from inadvertently being paid twice before it is returned to accounting.
Choice "b" is incorrect. Vouchers should be supported by a vendor's invoice and purchase order, but this does not prevent the same voucher from inadvertently being paid twice.
Choice "d" is incorrect. Vouchers should be prenumbered and accounted for, but this does not prevent the same voucher from inadvertently being paid twice.