Only d) is true.
Work Step by Step
a) This is false because an object can be slowing down as it goes upward. b) This is false as well. If I throw a ball straight up, it can have a straight path, but the acceleration will be downward. c) For the reason described in part a), this is false. d). Since the acceleration is downward, we see that if the ball only has horizontal velocity, it must gain vertical velocity. Thus, this statement is true.