Small airway leading to air sacs?

The correct answer is Bronchiole.

Bronchioles are the smallest branches of the bronchial tree, and they lead to the air sacs (alveoli) in the lungs. They are lined with ciliated columnar epithelial cells, which help to move mucus and foreign particles out of the lungs. Bronchioles also contain smooth muscle cells, which can contract or relax to change the size of the airway.