Final answer:
An unconfined aquifer is exposed to the surface and atmosphere, while a confined aquifer is isolated from the surface by impermeable materials. Permeability and porosity are key factors in determining good aquifers such as sandstone and limestone.
Step-by-step explanation:
The type of aquifer that has the water-bearing unit open to Earth's surface and atmosphere is known as a unconfined aquifer (A), while the type of aquifer that is separated from Earth's surface by low permeability rocks is called a confined aquifer (B).
Permeability is crucial for an aquifer as it allows for the transportation of fluids through the geologic material. The best aquifers are those made of rocks with high permeability and porosity such as sandstone and limestone. On the other hand, rocks such as shale or un-fractured igneous or metamorphic rocks serve as confining layers due to their low permeability, preventing water flow. An unconfined aquifer has its top marked by the water table, the boundary between the unsaturated and saturated zones where the pore spaces are completely filled with water.