Final answer:
The last two ribs, which are not attached to the sternum, are called floating ribs, which are ribs 11 and 12. The correct option is c.
Step-by-step explanation:
The last two (2) ribs that are not attached to the sternum are known as floating ribs. This is because these ribs, specifically ribs 11 and 12, do not attach to the sternum or to the costal cartilage of another rib. In contrast, true ribs are ribs 1-7 and are attached directly to the sternum via their costal cartilage.
The false ribs are ribs 8-12, with ribs 8-10 attaching indirectly to the sternum via the costal cartilage of the next higher rib. Therefore, the correct answer to the question is: c) floating ribs.