The answer is D. Oyster.
Tortoises, alligators, and snails are all reptiles. They are all cold-blooded animals with scales or scutes that cover their bodies. They all lay eggs, and they all breathe air.
Oysters, on the other hand, are mollusks. They are soft-bodied animals with a hard shell. They do not have scales or scutes, and they do not lay eggs. They reproduce by releasing sperm and eggs into the water, where the eggs are fertilized and develop into larvae.
Oysters are also filter feeders. They take in water through their gills and filter out small organisms to eat. Tortoises, alligators, and snails are all carnivores or omnivores. They eat a variety of plants and animals.
In conclusion, the answer is D. Oyster. Oysters are different from the other options because they are mollusks, not reptiles. They also have a different body structure and way of life.