As the air is slowly removed from the space around the ringing alarm clock, the appearance of the clock will not change significantly, but its sound will become fainter. This is because sound requires a medium (in this case, air) to travel through. As the air pressure decreases, the sound waves produced by the clock will find it increasingly difficult to propagate, leading to a decrease in volume. Therefore, the correct response would be:
"The appearance of the clock will not change, but its sound will become fainter."