Choice "D" is correct. When a maker signs a negotiable promissory note and when the note is negotiated to a holder in due course (or when an HDC transfers the note to a holder to whom the shelter doctrine applies), the maker must pay the note when due according to its terms when the maker signed unless the maker has a real defense. The facts here do not indicate that the maker has any real defenses.
Choice "b" is incorrect. The mere fact that the note was part of a transaction between merchants is not a reason to order the maker to pay it.
Choice "c" is incorrect. The note here appears to be negotiable – a writing, signed by the maker, containing an unconditional promise to pay a fixed amount of money at a definite time (90 days) with no unauthorized or other promises (making choice "a" also incorrect). The maker of a negotiable promissory note must pay the note when either (i) a holder who is not a holder in due course (and to whom the shelter doctrine does not apply) presents the note for payment and the maker has no defenses or (ii) a holder in due course (or a holder to whom the shelter doctrine applies) presents the note for payment and the maker has no real defenses (and the facts here do not indicate that any real defense applies).