Final answer:
The functional unit of the respiratory tree where gas exchange occurs is the alveoli, which are surrounded by capillaries for the exchange of oxygen and carbon dioxide.
Step-by-step explanation:
The functional unit of the respiratory tree where gas exchange occurs is the alveoli (option b). Alveoli are small sacs within the lungs at the end of the respiratory tree, surrounded by a network of capillaries. These structures are vital for exchanging oxygen and carbon dioxide between the bloodstream and the lungs.
The alveolar walls are extremely thin, which facilitates the diffusion of gases. Each alveolus is part of an alveolar sac, which is connected to the alveolar ducts, through which air travels after passing the respiratory bronchioles.