What defines an aquifer?

An aquifer is specifically defined as a geological formation that can store and transmit groundwater. The presence of sand and gravel allows for the accommodation of significant volumes of water, which is essential for the aquifer's function as a water supply. In such regions, porosity and permeability are critical, as they enable the stored water to flow through the material, making it accessible for extraction.

In contrast, other choices describe elements that do not fit the definition of an aquifer. For instance, impermeable rock does not store water effectively, as its lack of porosity prevents water from accumulating. Similarly, a layer of clay can restrict rather than facilitate water flow and would not function as an aquifer. Lastly, a body of surface water, like a river, is distinct from groundwater systems as it involves water found above the ground rather than stored within geological formations. Thus, the character of sand and gravel as the components that define an aquifer is what makes this choice correct.