Final answer:
The lower portion of the costal margin aligns with the umbilicus. The sternum, which includes the xiphoid process, is a central structure in the thoracic cage to which ribs attach.
Step-by-step explanation:
The lower portion of the costal margin is approximately at the same level as that of the umbilicus (option b). The sternum consists of the manubrium, body, and xiphoid process. The xiphoid process is the inferior tip of the sternum, which becomes ossified during middle age. Each rib is a curved bone that articulates with the thoracic vertebrae at the back and, in most cases, with the sternum at the front via costal cartilages. The sternum serves as a key structure for the attachment of the ribs and the clavicle, forming the thoracic cage that protects vital organs within the chest.