The correct answer is: C. By ticking down the moments, the clock reinforces the idea that time is running out for the Prince and his guests.
Indeed, it is clear that the clock is signaling the depleted time of life for Prospero and his callous guests. The fact that it is ebony, a very dark color reinforces this interpretation. Not only is the clock dark as death, but it is located in a black room which further underscores its ominous significance. Furthermore, this room happens to be the seventh room, and chronologically, the seventh day of the week is the last one, which clearly symbolizes that these people are living their last moments with each chime of the clock.