Explanation:

- Aquifer:

- An aquifer is a geological unit that is saturated with water.

- It can yield water in usable quantities to wells and springs.

- Supports the extraction of water at a rate sufficient for various needs.

- Correct Answer

- Reservoir:

- A reservoir is an artificial or natural storage space for water.

- Used for human consumption, irrigation, or hydroelectric power.

- Not necessarily capable of yielding water through natural percolation like aquifers.

- Estuary:

- An estuary is a coastal area where freshwater from rivers meets the ocean.

- Known for being rich in nutrients and biodiversity.

- Not related to underground water storage.

- Karst:

- A karst is a landscape formed from the dissolution of soluble rocks.

- Often contains caves and underground drainage systems.

- May have aquifers but is not defined by its ability to directly yield water to wells.