Which of the following best describes what alveoli are?
a.
they are the smallest tubes in the respiratory system.
b.
they have a small amount of surface area.
c.
they are found on the end of bronchioles.
d.
they are comprised of millions of cells.

The correct statement is- they are found at the end of the bronchioles.

Explanation:

Alveoli are vascularised air sacs that are formed by the terminal bronchioles. They are lined by simple squamous epithelium. Their primary function is to move air in and out of the cell. When we breathe, these alveoli expand because oxygen fills in these sacs and when we exhale the alveoli shrinks because of the removal of carbon dioxide from the alveoli.  

Further explanation:

Trachea divides into two branches and out of these two branches one enters into the right lung and another enters into the left lung and they are called primary bronchi. Then these primary bronchi divides and converts into a smaller secondary bronchi. They are also called lobar bronchi. These secondary bronchi again branch into tertiary bronchi which further divide into bronchioles. This whole branching of the trachea inside the lungs called bronchial tree. So bronchioles are terminal structure which give rise to many air sacs called alveoli which helps in exchange of oxygen and carbon dioxide between lungs and blood.  Alveoli are surrounded by blood capillaries which and exchange of carbon dioxide and oxygen occurs through diffusion.  As the partial pressure of carbon dioxide is higher in blood capillaries its diffusion takes place from blood to alveoli and due to higher partial pressure of oxygen in alveoli its diffusion takes place from alveoli to its surrounding capillaries.  

Learn more:

1. lungs (answered by JcAlmighty).

2. Exchange of gases  (answered by Riia).

Keywords:

Bronchioles,  alveoli,  blood capillaries, terminal bronchioles, carbon dioxide, oxygen, diffusion.

c. They are found on the end of bronchioles.

1.controls blood flow2.controls oxygen flow3.carries oxygen rich blood to veins