Question

one characteristic that is unique to water is that it

exists naturally in three states on earth.
dissolves very few substances.
can be changed from a liquid to a solid.
has a low specific heat.

Explanation:

Water is unique as it naturally exists in three states (solid - ice, liquid - water, gas - water vapor) on Earth. Many substances dissolve in water (it's a universal solvent), substances can change from liquid to solid (e.g., freezing of other liquids too), and water has a high specific heat (not low).

Answer:

exists naturally in three states on Earth.