What geological feature stores groundwater and is characterized by its porosity?

An aquifer is a geological formation that is specifically designed to store and transmit groundwater due to its porosity and permeability. It consists of materials, such as sand, gravel, and certain types of rock that can hold significant amounts of water and allow it to flow through the voids between particles. This ability to store groundwater makes aquifers critical for supplying water to wells and springs, as well as sustaining surface water bodies.

In contrast, limestone can act as a type of aquifer due to its natural fractures and solution cavities that can hold water. However, not all limestone formations have the necessary properties to qualify as productive aquifers. Riverbeds may store some water in sandy or gravelly substrates but are primarily conduits for surface water flow. Lakes, while they contain water, do not store groundwater and are not relevant in terms of groundwater resources; rather, they are surface water bodies.

Thus, the defining characteristics of an aquifer, including its capacity for both storing and transmitting groundwater effectively, make it the correct answer to this question.