Final answer:
Members of Chondrichthyes, such as sharks and rays, are thought to have descended from ancient fishes with bony skeletons. The correct answer is option a).
Step-by-step explanation:
Members of Chondrichthyes, which are cartilaginous fishes like sharks, rays, and skates, are thought to have descended from fishes that had bony skeletons. These ancient ancestors are believed to be the placoderms, which had endoskeletons made of bone.
Over time, Chondrichthyes evolved to have skeletons made primarily of cartilage, which is a lighter and more flexible structure compared to bone. This evolution likely provided adaptive advantages in their marine environments.
Although some parts of their skeletons are strengthened by granules of calcium carbonate, it is important to note that this is not the same as having a bony skeleton. Additionally, Chondrichthyes are characterized by having jaws, paired fins, and a cartilaginous structure instead of bones.