Final answer:
The first portion of the respiratory tree where gas exchange occurs is the respiratory bronchiole, marking the beginning of the respiratory zone.
Step-by-step explanation:
The first portion of the respiratory tree where gas exchange can occur is the d. Respiratory bronchiole. The respiratory bronchioles are the smallest airways of the lungs that contain alveoli, where gas exchange takes place. They are the beginning of the respiratory zone, contrasting with the conducting zone which includes all other components leading up to the respiratory bronchioles, such as the terminal bronchioles, which do not participate in gas exchange. Following the respiratory bronchioles, the airway continues into the alveolar ducts, and then to the alveolar sacs, where the alveoli are densely packed for efficient gas exchange.