Final answer:
The structure in the lower respiratory tract that lacks cartilage but has a significant amount of smooth muscle is the bronchioles (B).
Step-by-step explanation:
The structure of the lower respiratory tract that lacks cartilage but contains a large amount of smooth muscle is the Bronchioles (B).
Unlike the bronchi and trachea, which contain cartilage to maintain their structure, the bronchioles are supported by elastic fibers and rely on inhaled air to keep their shape. As the respiratory passages decrease in diameter and branch off into bronchioles, the amount of smooth muscle relative to the diameter of the airways increases, allowing for control of airflow through muscle contraction or relaxation.