The correct answer is A. Fishes.
Seahorses are fish, and they belong to the class Actinopterygii, which also includes bony fish, such as salmon, tuna, and trout. Seahorses are unique among fish in that they have a long, pipe-like snout and a prehensile tail that they use to grip onto seaweed and other underwater plants. They are also unusual in that the male seahorse carries the eggs in a pouch on his stomach until they hatch.
Option B is incorrect because mammals are warm-blooded animals that give birth to live young and nurse their babies with milk. Seahorses are cold-blooded animals that lay eggs.
Option C is incorrect because reptiles are cold-blooded animals that have scales or scutes on their skin. Seahorses do not have scales or scutes.
Option D is incorrect because mollusks are soft-bodied animals that have a hard shell. Seahorses do not have a hard shell.