Choice "C" is correct. An instrument is payable at a definite time if it can be established from the face of the instrument when the obligation will become due. An obligation payable 15 days after sight is payable 15 days after it is presented for payment.
Choice "b" is incorrect. Although six months is a definite time, the option of the payor to extend indefinitely the time for payment destroys negotiability.
Choice "a" is incorrect. If an instrument is not dated, we cannot know when 30 days after date is. Therefore, this is not payable at a definite time.
Choice "d" is incorrect. Although the payee will die some day, we do not know when, so the date of payment is not definite.