The smallest branches of the bronchial tree are:
a. primary bronchi.
b. secondary bronchi.
c. tertiary bronchi.
d. bronchioles.

The correct answer is D.

Explanation:

The trachea divides into 2 main primary bronchi (left and right bronchus). Then the right bronchus divides into 3 secondary bronchi (or lobar bronchi) and the left bronchus does it into 2 secondary bronchi.

All of the secondary bronchi divide into tertiary bronchi, also called segmental bronchi.

After that, the tertiary bronchi continue division to several branches until they become bronchioles (the smallest branches of the bronchial tree).

The bronchioles divide into terminal bronchioles, then respiratory bronchioles, then alveolar ducts and finally alveolar sacs, where gas exchange occurs.