Answer:
The correct answer is option A. "skeleton type and skin covering".
Explanation:
The characteristics that define a fish under agnatha, cartilaginous fishes, or bony fishes classes are related to the skeleton type and skin covering properties of the fish. For instance, agnatha fishes are jawless, cartilaginous fishes (Chondrichthyes) have skeletons made of cartilage rather than bones, while bony fishes (Osteichthyes) as their name implies, have skeletons primarily composed of bone tissue. Therefore, the skeleton type and skin covering would be the best pair of characteristics to chose in order to place a fish in one of the three classes.
