An aquifer is best described as which of the following?

• A. A geological formation that can store and transmit water, often tapped for human use.
• B. A type of rock that resists erosion.
• C. A channel carved by water to transport sediment.
• D. A mountain formation with many caves.

Answer: (A)

An aquifer is a body of permeable rock or sediment that stores groundwater and allows it to move through its pore spaces or fractures. This storage-and-transmission property is what makes aquifers useful sources of water, since water from precipitation percolates down, fills the spaces, and can be tapped by wells or springs. The other descriptions don’t capture this combination: a rock that resists erosion isn’t about storing or transmitting water; a channel carved by water describes surface flow, not underground storage; and a mountain with caves points to karst features, which may involve caves but not the essential groundwater-realm storage and movement.