The correct answer is (d), Limulus. Limulus is a horseshoe crab, a marine arthropod that has existed for over 450 million years. It is one of the few living members of the group Xiphosura, which also includes extinct species such as the giant sea scorpions. Limulus is often referred to as a “living fossil” because it has changed very little in appearance over millions of years. It is an important part of the marine ecosystem, and its blood is used in medical research.
Balanus is a genus of barnacles, which are crustaceans that attach themselves to hard surfaces. They have a hard outer shell and a soft inner body. Barnacles are found in all oceans, and they can live in a variety of habitats, including on rocks, ships, and whales.
Daphnia is a genus of water fleas, which are small crustaceans that live in freshwater. They are filter feeders, and they eat algae and other small organisms. Daphnia are an important part of the food web in freshwater ecosystems.
Sarcoptes is a genus of mites, which are small arachnids. They are parasites, and they feed on the skin of mammals, birds, and reptiles. Sarcoptes mites can cause a skin condition called scabies.
Limulus is the correct answer because it is a “living fossil”. It has changed very little in appearance over millions of years, and it is an important part of the marine ecosystem.