Choice "C" is correct. Negotiable commercial paper must be payable on demand or at a definite time [UCC 3-104(1)(c)]. An instrument payable at someone's death or at a time after someone's death is not payable at a definite time because while all people will die, we don't know when [UCC 3-109(2)].
Choice "a" is incorrect. Negotiable commercial paper must be payable on demand or at a definite time [UCC 3-104(1)(c)], and an instrument payable at a definite time but subject to acceleration is considered to be payable at a definite time because only the latest date for payment need be known [UCC 3-109(1)(c)].
Choice "b" is incorrect. An instrument is not made non-negotiable by postdating [UCC 3-114(1)].
Choice "d" is incorrect. While to be negotiable an instrument must not be subject to any unauthorized promises [UCC 3-104(1)(b)], the UCC authorizes promises to maintain collateral [UCC 3-112(1)(c)].
