C.In the ocean
Explanation: Approximately 97.5% of the Earth's water is found in the saltwater of the oceans and seas. The remaining 2.5% is freshwater, with a significant portion of it stored as ice in glaciers and polar ice caps, which corresponds to option D. While there is also water underground (option B) and in freshwater lakes (option A), the vast majority is in the form of saltwater in the oceans.