Which statement best describes how the structure of the alveoli allows the lungs to function properly

Out of the following given choices;

a. they increase the amount of energy transferred from the lungs to the blood

b. they increase the flexibility of the lungs as they expand during inhalation

c. they increase the volume of the lungs, allowing more oxygen to be inhaled

d. they increase the surface area of the lungs, allowing the efficient gas exchange

The answer is D. Due to their microscopic nature, they are numerous in the lungs and are also circular-shaped so as to increase the surface area for gaseous exchange. Other characteristics of alveoli that increase gas exchange are that they thinned-walled (with only a layer of cells) and moist.