Adaptations to terrestrial environments evolved independently in arthropods, mollusks, nematodes, and annelids. What logical conclusion would you make from this statement?
Any animal that lives in water and has special features to live, hunt, breathe and breed in the water.
Any animal which lives in water throughout their life or most of it is an aquatic animal. It can be amphibians like frogs or poriferans like spongilla. All fishes are aquatic as they all live in water, Cyclostomata, Chondrichthyes, and Osteichthyes are all aquatic
Mammals belonging to Order Cetaceans like whales and dolphins are also aquatic beings
Hence, the right answer is "the common ancestor of all these groups was probably aquatic"
