The correct option is
B Sea Horse
A. Sea Horse (Hippocampus) is a fish belonging to class Osteichthyes under phylum Chordata.
B. Sea cow (Hydrodamalis) is a large aquatic mammal under phylum Chordata.
C. Sea cucumber (Holothuria) is a marine animal under phylum Echinodermata.
D. Sea urchin (Echinus) is a spiny animal under phylum Echinodermata.
Hence, sea horse is a fish. So, the correct answer is 'Sea Horse'.