Which cavity extends from the base of the skull to the lower back and contains the spinal cord?

The spinal cavity, also known as the vertebral or spinal canal, is the correct answer because it specifically encompasses the area that extends from the base of the skull down to the lower back, providing a protective passage for the spinal cord. This cavity is formed by the vertebrae of the spine and the surrounding structures, ensuring that the spinal cord, a critical component of the central nervous system, is safeguarded against injury.

In contrast, the cranial cavity houses the brain; the thoracic cavity contains the heart and lungs; and the pleural cavity surrounds each lung specifically. Each of these cavities serves distinct functions and houses different organs. The spinal cavity, with its unique structure and function, is solely focused on protecting the spinal cord and facilitating its essential role in transmitting nerve signals between the brain and the rest of the body.