Choice "A" is correct. To be negotiable, an instrument must be payable at a definite time. "10 days after Train sells his car" is not a definite time − we do not know when or if the car will be sold. Therefore, the instrument is not negotiable.
Choice "d" is incorrect. To be negotiable, an instrument generally cannot state that it is subject to another writing. However, nothing in the facts indicates that Train's note said that it was subject to another writing.
Choice "b" is incorrect. To be negotiable, an instrument must be for a fixed amount of money, but that an instrument is for a "sum certain" alone is not sufficient to make the instrument negotiable. The other requirements for negotiability must be met as well, and here the note is not negotiable because it was not payable at a definite time.
Choice "c" is incorrect. Whether a note is negotiable is determined from the face of the instrument (i.e., whether it is conditional depends on what the instrument says). A condition in the instrument is not cured merely because the condition has been fulfilled.