A. Snails are mollusks and have no spine in their bodies, so they are invertebrates; manatees are gastropods and have no spine in their bodies, so they are invertebrates.

B. Abalone is a mollusc, with no spine in its body, and is an invertebrate; shark is a fish, with a spine in its body, and is a vertebrate.

C. Leeches are annelids and have no spine in their bodies, so they are invertebrates; Hydras are coelenterates and have no spine in their bodies, so they are invertebrates.

D. Zebras are mammals, with spines in their bodies, and are vertebrates; seahorses are fish, with spines in their bodies, and are vertebrates.

So choose: D