The correct option is D Gills
Crustaceans form a very large group of arthropods,. Arthropods is a group of organisms which have jointed legs. This group includes crabs, lobsters, crayfish, shrimp, krill etc. In many smaller crustaceans, respiration occurs through general body surface. In other crustaceans, gills are the main respiratory organs. They are also involved in excretory function. These gills are generally associated with the appendages. Gills in crustaceans originate as an out pushing of the body wall and in larger forms, occur in series.
Thus, the correct answer is option D.