An aquifer is defined as:

• A. A surface freshwater lake.
• B. A wetland that stores groundwater.
• C. Underground caverns and porous layers of sand, gravel, or rock in which groundwater is stored.
• D. A saltwater estuary near the coast.

Answer: (C)

An aquifer is an underground layer of permeable material—such as sand, gravel, or certain rocks—that stores and transmits groundwater. Water fills the pore spaces in this rock or sediment and can move slowly through it, allowing wells to draw freshwater up to the surface. This description specifically captures the idea of underground storage and the ability to yield water, which is why it matches the concept of an aquifer. In contrast, a surface lake sits on the surface, a wetland stores groundwater but isn’t defined by an underground, well-yielding layer, and a saltwater estuary near the coast involves tidal mixing of surface waters, not groundwater storage.