Which of the following organisms belong to the phylum Cnidaria?

The organism that belongs to the phylum Cnidaria is the sea anemone. Cnidarians are characterized by their unique features, including the presence of specialized cells called cnidocytes, which house stinging structures used for defense and capturing prey. Sea anemones, along with jellyfish and corals, exhibit these key characteristics, making them true representatives of this phylum.

In contrast, flatworms belong to the phylum Platyhelminthes, which are characterized by their flat bodies and lack of specialized respiratory and circulatory systems. Roundworms are part of the phylum Nematoda and possess a distinctly cylindrical body structure with a complete digestive system. Vertebrates belong to the phylum Chordata, defined by the presence of a backbone and a higher level of complexity. Thus, the sea anemone stands out as the sole representative from the phylum Cnidaria in this grouping.