Choice "A" is correct. By stamping the voucher "paid," the check signer cancels the voucher so it cannot be resubmitted for payment.
Choice "c" is incorrect. Even invoices that are supported by prenumbered sales invoices can be resubmitted for payment if they are not canceled, resulting in duplicate payments.
Choice "b" is incorrect. Accounting for the sequence of prenumbered vouchers would only test whether all vouchers are present. It would not prevent a voucher from being paid twice.
Choice "d" is incorrect. Proper authorization would help ensure that payments were properly authorized, but would not prevent duplicate payments.