The right option is; A. Alligator
You would expect to find an alligator in a marsh.
Alligators are crocodilians, and large animals that belongs to the genus; Alligator, and family; Alligatoridae. A. mississippiensis and A. sinensis are the two living species of this group. Alligators have a broad snout, and are carnivorous. Alligators can be found in brackish and fresh water environments such as marshes, lakes, swamps, rivers and ponds.