The correct option is
C Osteichthyes
A. Agnatha is a superclass of jawless fishes.
B. Chondrichthyes is a class of cartilaginous fishes and includes sharks, rays, and Chimaera.
C. Osteichthyes is a class of bony fishes and includes lungfishes, ray-finned and lobe-finned fishes.
D. Mammals are terrestrial and warm-blooded animals that have characteristic mammary glands.
Hippocampus, commonly known as Seahorse, belongs to class Osteichthyes.
Thus, the correct answer is 'Osteichthyes'.