The correct option is option 3: Hydra and sea anemone. Coelenterata, also known as Cnidaria, is a phylum of organisms that includes animals such as jellyfish, corals, sea anemones, and hydra. These organisms are characterized by the presence of specialized cells called cnidocytes, which contain stinging structures called nematocysts.

Option 1, Planaria and liver fluke, belong to the phylum Platyhelminthes, not Coelenterata. Planaria are flatworms, while liver flukes are parasitic flatworms.

Option 2, sea urchin and sea cucumber, belong to the phylum Echinodermata. These organisms are characterized by a spiny skin and radial symmetry, while Coelenterata typically have a more jelly-like body structure and radial symmetry.

Option 4, Euplectella and sycon, are both types of sponges and belong to the phylum Porifera. Sponges are multicellular organisms that filter water to obtain food.

In conclusion, the organisms that belong to Coelenterata are hydra and sea anemone.