Answer: Option B. "respiratory bronchioles"
Step-by-step explanation:
Alveoli are the air sacs presents in the end of the respiratory system in the lungs which are responsible for the exchange of oxygen and carbon dioxide to and from the bloodstream.
Respiratory bronchioles are the first passage of respiratory zone that contains alveoli in their walls.
The trachea in lungs gives rise to two primary (main) bronchii which branches into secondary and tertiary bronchii. These bronchii gives rise to bronchioles and the smallest is terminal bronchioles. Terminal bronchioles on further division give rise to respiratory bronchioles, which leads to the tiny air sacs called alveoli, consist of an epithelial layer and surrounded by capillaries.
Hence, the correct option is B.