11th and 12th pair of ribs which are imperfectly formed and do not reach the sternum are called

C) Floating ribs
The 11th and 12th pairs of ribs are called floating ribs because they do not have any attachment to the sternum or cartilage. These ribs are attached only to the vertebral column and provide some protection to the kidneys but are more flexible, allowing for the expansion and movement of the chest during breathing.