Question

these structures of the respiratory tree have no cartilage, but have significant smooth muscle in their walls. lobar bronchi segmental bronchi alveoli bronchioles

Explanation:

Lobar and segmental bronchi have cartilage in their walls. Alveoli are tiny air - sacs with no significant smooth muscle. Bronchioles are small airways in the respiratory tree that lack cartilage and have a significant amount of smooth muscle in their walls which can constrict or dilate to control airflow.

Answer:

Bronchioles