The option stating that the largest proportion of remaining freshwater exists as groundwater is correct because groundwater constitutes about 30.1% of the Earth's freshwater resources, surpassing surface water sources like rivers and lakes. This vast reservoir of freshwater is critical for human consumption, agriculture, and ecosystem services, highlighting its importance in sustaining life and supporting economic activities. Groundwater depletion is a significant concern, making its management essential for water security, particularly in arid and semi-arid regions.