Final answer:
The lungs appear black on a CT axial section through the thorax.
Step-by-step explanation:
The structure or structures that appear black on a CT axial section through the thorax are the A) Lungs.
The lungs appear black on a CT scan because they are filled with air, which absorbs X-rays, resulting in less radiation being detected by the scanner. This contrast makes the lungs appear dark or black on the image.