Trachea is also called windpipe which is a hollow structure made up of approximately 20 cartilaginous rings. It connects the larynx to the bronchi of the lungs. It helps in the passage of oxygen to the lungs.
Trachea divides into two primary bronchi at the level of the 5th thoracic vertebra. One primary bronchi enter into right lung and another into the left lung. Then these primary bronchi divides into secondary bronchi in the lungs.
Then secondary branched divides into tertiary bronchi and tertiary bronchi branched into smaller bronchioles. These smaller bronchioles branch further to form terminal bronchioles. So the correct answer is D. trachea.
