Final answer:
The reservoir with the longest residence time of hundreds of thousands of years in the hydrologic cycle is the ice caps (C). While glaciers and ice caps generally hold water ranging from 1,000 to 10,000 years, ice caps can contain water for much longer periods. The oceans and groundwater have shorter residence times.
Step-by-step explanation:
The hydrologic cycle involves the movement of water among various reservoirs, each with its own residence time. Residence time is defined as the average time an individual water molecule remains within a particular reservoir. Among the options provided, the reservoir with the longest residence time of hundreds of thousands of years is ice caps (C). According to the given information, glaciers and ice caps have an average residence time for water molecules ranging from 1,000 to 10,000 years, but some of this water can remain frozen in ice caps for much longer, up to hundreds of thousands of years.
In contrast, the ocean, while being the largest reservoir of water, has an average residence time of approximately 4,000 years, which is significantly shorter. Groundwater can have a residence time that varies widely, from two weeks to 10,000 years, depending on the depth and other factors. Finally, the atmosphere has the shortest residence time of just a few days.
The majority of Earth's freshwater is contained in glaciers and ice caps, with only a small fraction being available as usable fresh water in the form of shallow groundwater, lakes, and rivers.