Final answer:
Option (1), Crustaceans like shrimp and lobsters are primarily recognized for their exoskeleton, which is a universal feature among such organisms. This exoskeleton is shed during molting to allow growth.
Step-by-step explanation:
Crustaceans, such as shrimp and lobsters, are recognized by their possession of several characteristic features. However, when specifically asked to choose between options like an exoskeleton, gills, antennae, and swimming legs, the most universally recognizable feature of crustaceans is their exoskeleton.
Crustaceans, which include organisms like crabs, lobsters, shrimp, and krill, all have an exoskeleton that may be bound together in sections like the carapace. This hard exoskeleton is periodically shed in a process called molting to allow the animal to grow, a phase during which they are very vulnerable.
Crustaceans also possess other distinctive features such as two pairs of antennae, mandibles for mouthparts, biramous appendages, and an open circulatory system. They typically have separate sexes, although some crustacean species exhibit hermaphroditism. Most are aquatic, but some like the pill bugs are terrestrial.