Answer:
Water
Step-by-step explanation:
If we consider the planet to be Earth, it is water. It is the only item that exists in all three states of matter: Water (Liquid state), Ice (Solid state) and water vapor (Gaseous state) at standard pressure. No other elements/compound can exist in all three states at standard pressure. In fact, Earth is the only known planet to have this condition which makes it suitable for life.