The correct answer is alveoli.
At the end of the respiratory tree, alveoli are located. These are tiny, balloon-shaped air sacs are arranged in clusters throughout the lungs. these are the site of gaseous exchange as alveoli are surrounded by thin pulmonary capillaries which allows exchange of gases. The oxygen diffuse into the blood capillaries and the carbon dioxide diffused into the alveolar space from the blood capillaries.