Answer:
d. underground aquifers
Explanation:
The water on our planet is not mostly freshwater, but instead it is almost entirely saline water, with the freshwater reserves representing only around 2.5% of the total water reserves. From that water, the majority, around 67.8% is frozen, being found in the ice caps and glaciers. Around 30.1% is found in the underground aquifers, and only 1.2% is surface and other freshwater.
The groundwater aquifers are found on all continents. Some are very large, while some are small. Because they are such large reservoirs of freshwater, the humans have started to use them intensively in the past few decades. Unfortunately, their usage has not been planned well, as much more water is extracted from them than what is renewed naturally. This leads to depletion of the ground waters, and lot of aquifers are now in danger of getting totally dry, which in turn will cause huge problems in numerous regions around the world.
