What structure extends from the base of the skull to the lower back and houses the spinal cord?

The spinal cavity is the correct answer because it specifically refers to the canal formed within the vertebral column that extends from the base of the skull down to the lower back. This cavity houses the spinal cord, which is a critical part of the central nervous system, responsible for transmitting signals between the brain and the rest of the body.

The cranial cavity, while important, is limited to the space within the skull that contains the brain and does not extend down the vertebral column. The thoracic and abdominal cavities refer to regions within the trunk of the body that contain various organs but do not enclose the spinal cord. Understanding the distinctions between these anatomical structures is essential for recognizing how they contribute to the overall framework and function of the human body.