Choice "A" is correct. Any writing to be a negotiable instrument must be payable to order or to bearer, with the exception of checks. If the instrument is payable to order, it is negotiated by delivery with any necessary endorsement; if payable to bearer, it is negotiated by delivery alone. UCC 3-104
Choice "b" is incorrect. Whether an instrument is negotiable is determined by its form when drawn or made; the subsequent signature of a payee can neither create nor destroy negotiability.
Choice "d" is incorrect. A negotiable instrument need not contain references to any other document.
Choice "c" is incorrect. If an instrument is conditional, it generally cannot be negotiable.