The correct option is
C Bony fish
A. Coelenterates are the acoelomate animals that are marine, aquatic, sessile or free swimming.
B. Bony fishes belong to class Osteichthyes. These fishes have cartilaginous endoskeleton in the embryonic stage which is replaced by bones in adult forms.
C. Mammals are the warm-blooded animals that have mammary glands.
D. Reptiles are the animals that creep on the land.
Sea Horse (Hippocampus) is a bony fish. So, the correct answer is 'Bony fish'.