Final answer:
The tiny sacs at the end of bronchioles are called alveoli, where the gas exchange between the lungs and blood takes place.
Step-by-step explanation:
The tiny sacs found at the end of the bronchioles are called alveoli (option b). These are the structures where the exchange of gases between the lungs and blood occurs. The terminal bronchioles subdivide into microscopic respiratory bronchioles, which further divide into several alveolar ducts. These ducts lead to alveolar sacs, each containing 20 to 30 alveoli. These alveoli, which resemble tiny bubbles or a bunch of grapes, are in direct contact with the capillaries of the circulatory system allowing for the efficient exchange of oxygen and carbon dioxide.