Aquifers are
a. structures used to transport water from city to city
b. created through evapotranspiration
c. densely packed snow close to becoming glaciers
d. layers of porous rock or sand

The correct answer is letter D. layers of porous rock or sand. Aquifers are layer form part of the underground or groundwater. It is permeable that is capable of absorbing more water. Thus, it is one of the important parts of earth's layer because it is where water deposits are located. Using a water well, waters can be extracted.