Which of the following structures are regions of the lung?

A. Frontal, maxillary, sphenoidal, ethmoidal
B. Eustachian, pharynx, bronchi
C. Cilia, adenoids, mucous, epiglottis
D. Apex, base, hilum

To determine which of the following structures are regions of the lung, let's go through each option:

A. Frontal, maxillary, sphenoidal, ethmoidal: These structures are actually regions of the skull, not the lung.

B. Eustachian, pharynx, bronchi: While the pharynx and bronchi are related to the respiratory system, the Eustachian tube is part of the ear and not directly associated with the lung.

C. Cilia, adenoids, mucous, epiglottis: Among these structures, only the cilia and mucous are associated with the lung. Cilia are tiny hair-like structures that line the respiratory tract and help move mucus and foreign particles out of the lungs. Mucus, on the other hand, is a sticky substance produced by cells in the respiratory tract to trap dust, bacteria, and other particles. Adenoids and epiglottis are not regions of the lung but rather separate structures in the respiratory system.

D. Apex, base, hilum: These are indeed regions of the lung. The apex refers to the uppermost portion of the lung, while the base is the lowermost part. The hilum is a specific region of the lung where the bronchi, blood vessels, and lymph vessels enter and exit.

Therefore, the correct answer is: D. Apex, base, hilum