Which of the following fish is jawless?
A. Chondrichthyes
B. Osteichthyes
C. Cyclostomata
D. Amphibia

Hint: The gill-bearing aquatic animals that lack limbs with digits are called fishes. They include cartilaginous, hagfish, lampreys, and bony fishes. The jawless fish means the fish species which lack true jaw or defined jaw structure. These are lampreys and hagfish. They have horny epidermal structures in their mouth.

Complete answer: Fishes are a large group of marine animals that lack limbs with digits, rather they have fins. They have gills in place of lungs to breathe deep inside the water. They have either a cartilaginous or body structure. Most of the fishes are ectothermic which allows them to change their body temperatures according to their surroundings. Most of the fishes have jaws with varied structures that enable them to have varied food. There are some jawless fishes like lamprey and hagfish.
A) Chondrichthyes are cartilaginous vertebrate fishes having defined jaws. They have paired fins and a skeleton made of cartilage rather than bone.
B) Osteichthyes that is also known as bony fish forms a diverse group of fishes that bear skeletons primarily made of bone tissue. They have a stable pattern of cranial bones. A rooted, medial insertion of mandibular muscles is seen in the lower jaw of these fishes.
C) Cyclostomata which means round mouth fishes are a group of agnathans. They comprise jawless fishes that have horny epidermal structures resembling the function of teeth. Unlike jawed fishes, these fishes have branchial gills positioned internally.

D) Amphibia fishes are also jawed fishes. They have upper jawbones fused to form a braincase. The lower jawbones form a mandible.
Hence, option C is the correct answer.

Note: Other differences seen in jawless fishes are lack of thymus, myelin, spleen and sympathetic ganglia. They also lack internal eye muscles. They have a single olfactory organ with a single nostril. The Lampreys have tooth plates on their tongue-like structures. The hagfish have cartilaginous plates on the floor of their mouth.