Which of the following is the largest reservoir of potable water?

The correct answer is option 2. Groundwater.

Nearly 71% of the earth's surface is covered with water. Of this, 3.5% is fresh water and 96.5% of the water is in the seas. The remaining fresh water is found in river ponds or lakes. Of the total amount of fresh water, 2.5% is contained in glaciers as ice, making it unusable for human use. Therefore, the biggest source of drinkable water is groundwater.