The cluster of air sacs on the lungs is called
(a)Alveolus
(b)Bronchi
(c)Bronchioles
(d)Air spaces

Hint They are balloon-like air sacs located at the distal ends of the bronchial tree. Approximately, there are 700 million alveoli in each lung, where they facilitate the gaseous exchange of oxygen and carbon dioxide between inhaled air and the bloodstream.

Complete answer:
The dimensions of the pulmonary alveolus are roughly 0.2 to 0.5 mm in diameter. These alveoli are located at the terminals of air passageways in the lungs. Air that enters the lungs during inhalation, travels through numerous passageways called bronchi and then flows into the alveoli. Each alveolus is surrounded by various thin branches of blood capillaries that are supplied by minor branches of the pulmonary artery.
The barrier between alveolar air and blood capillary creates a respiratory membrane that consists only of a squamous alveolar cell, a squamous endothelial cell of the capillary, and a shared basement membrane.

Additional information:
The concentration of the incoming oxygen is more in the alveolus than in the red blood cells during inhalation. Therefore, oxygen leaves the alveolus and subsequently enters the red blood cells. The opposite occurs during exhalation. This time the concentration of carbon dioxide is lower in the alveolus than in the red blood cells, thus, carbon dioxide leaves the red blood cell, enters the alveolus, and is exhaled.
Alveolus mainly consists of three types of cells:
-Type I pneumocytes cover most of the interior surface. These cells are very thin and mainly contain squamous epithelium cells that are ideal for gas exchange.
-Type II pneumocytes are much less prevalent in each alveolus and found in between type I pneumocytes. These cells are large and cuboidal and also have apical microvilli.
-Alveolar macrophages are mononuclear phagocytes that are found in alveoli. In addition to this, they derive from blood monocytes.

So, the correct answer is Alveolus.

Notes: Extensive lymphatic drainage than any other organ in the body can also be found in the lungs. Since edema (buildup of fluid) can cause many pathophysiologies, some of which are often fatal. Also, low capillary blood pressure also prevents the rupture of the delicate respiratory membrane.