Solution

Sea horse is a fish.
Sea mouse is a marine polychaete worm.
Sea cow is now an extinct mammal.
Sea hares are medium-sized to very large Opisthobranchia. Sea hare is commonly known as sea slug. They belong to the group molluscs. They are marine gastropods. They have soft internal shell made up of protein. These are molluscs in the superfamilies Aplysioidea and Akeroidea.
Thus, the correct answer is option C.