Which of the following is defined as a marine coelenterate with a soft, gelatinous structure?

The term "marine coelenterate" refers to organisms belonging to the phylum Cnidaria, which includes jellyfish, corals, and sea anemones. Jellyfish are characterized by their soft, gelatinous bodies, which primarily consist of a substance called mesoglea. This structure allows them to drift with ocean currents and gives them their characteristic bell shape. They possess tentacles equipped with specialized cells called nematocysts, which are used for capturing prey and defense.

In contrast, starfish, corals, and seaweeds have different structural features. Starfish (or sea stars) belong to the phylum Echinodermata and have a hard, calcified body covered with skin. Coral, while also a coelenterate, has a hard exoskeleton and is more rigid than a jellyfish. Seaweed, on the other hand, is a type of algae and does not belong to the phylum Cnidaria; its structure is entirely different as it is composed of plant-like tissues that can be more fibrous or leafy in appearance.

Thus, the defining characteristics of jellyfish make it the correct answer in identifying a marine coelenterate with a soft, gelatinous structure.