Which best describes the amount of Earth’s total water supply that is usable freshwater?

The amount of Earth's total water supply that is usable freshwater is very small. Here's a step-by-step breakdown:

1. Total water on Earth: This includes all the water in the oceans, ice caps, lakes, rivers, groundwater, atmospheric water, and even the water in you, your dog, and your tomato plant.

2. Saltwater vs Freshwater: About 97.5% of all water on Earth is saltwater, leaving only 2.5% as freshwater.

3. Accessible Freshwater: However, not all of this freshwater is easily accessible or usable. Over 68% of it is trapped in ice caps and glaciers.

4. Groundwater: About 30% of the world's freshwater is stored underground in the form of groundwater. But again, not all of this is easily accessible.

5. Usable Freshwater: Only about 0.3% of our freshwater is found in the surface water of lakes, rivers, and swamps. This is the water that is most easily accessible and usable for human needs.

So, the best description for the amount of Earth’s total water supply that is usable freshwater would be "a very small fraction".