Of the following structures, the largest in diameter is the ________.

A) primary bronchus
B) secondary bronchus
C) respiratory bronchiole
D) trachea
E) tertiary bronchus

Primary Bronchus

The major airway in the lung that branches from the trachea to each lung, conducting air into the lung's lobes.

Trachea

The windpipe, a tube connecting the larynx to the bronchi, allowing the passage of air to the lungs.