Final answer:
The ribs extend from the spine (thoracic vertebrae) to form the rib cage, which encloses and protects the upper part of the body, including vital organs such as the heart and lungs.
Step-by-step explanation:
The ribs are curved bones extending from the thoracic vertebrae and enclosing the upper part of the body. Therefore, the correct answer is a) spine. The ribs form the rib cage, which includes the sternum (or breastbone) as well as 12 pairs of ribs that attach at joints to the thoracic vertebrae.
The ribs are divided into true ribs, false ribs, and floating ribs. True ribs attach directly to the sternum, false ribs attach to the sternum via cartilage connected to other ribs, and the floating ribs do not attach to the sternum at all. This structure of the rib cage serves to protect vital organs such as the heart and lungs and provides support for the upper body.