What structure protects the brain?

The skull, also known as the cranium, is the primary structure that protects the brain. It is composed of several bones that are fused together to form a rigid barrier around the delicate tissues of the brain, safeguarding it from physical trauma, infections, and environmental factors. The cranial bones also provide a stable structure where the brain can reside, while allowing for some flexibility to accommodate growth and movement.

While other structures listed, such as the spine and ribcage, do have protective roles for other vital organs in the body (such as the spinal cord and thoracic organs), they are not directly involved in the protection of the brain. The sternum, being part of the ribcage, similarly does not pertain to cranial protection. Therefore, the skull is specifically designed to surround and protect the brain, making it the correct answer in this context.