Final answer:
The moveable projections from the surface of cells in the respiratory tract are cilia. These hair-like structures are responsible for moving mucus and trapped particles upward toward the throat to keep the airways clear.
Step-by-step explanation:
The moveable projections from the surface of cells found in abundance in the respiratory tract are cilia. Cilia are microscopic, hair-like projections that extend from the apical cell membrane and are supported by microtubules. These ciliated cells beat in a coordinated fashion to move mucus mixed with dust, germs, and other foreign particles away from the lungs and toward the throat, which is an action fundamental to keeping the respiratory system clean.
In the context of the given options, A) Microvilli are small projections that increase the surface area of cells but do not move. B) Cilia are the correct answer as they are responsible for moving substances across the cell surface and are abundant in the respiratory tract. C) Flagella are long and fewer in number used for the propulsion of the cell itself, such as in sperm cells. D) Stereocilia are elongated microvilli that do not beat like cilia but increase cell surface area and facilitate absorption. Therefore, the matching term for moveable projections in the respiratory tract is cilia.