Which ancient timekeeping devices utilized water?

The clepsydra, or water clock, is the correct answer because it specifically measures time through the regulated flow of water. This ancient device relies on the principle that a certain amount of water would flow from one container to another at a consistent rate. As water fills or empties from the container, it allows users to keep track of time intervals based on the amount of water that has passed.

In contrast, sundials function by using the position of the sun's shadow cast on a flat surface, making them dependent on sunlight rather than any material substance to measure time. Fire clocks measure time through the burning of materials, often using a length of string or other items that burn at a steady rate. Sand timers, or hourglasses, track the passage of time through the flow of sand from one chamber to another but do not involve water in their operation. Each of these alternatives operates on different principles, making the clepsydra unique as a water-based timekeeping device.