The correct option is D Obelia
Cnidarians exhibit two types of body forms. They are polyp and medusa forms. The Polyp form is sessile and cylindrical like Hydra, Adamsia, whereas the medusa is umbrella-shaped and free-swimming, for example, Aurelia or jellyfish. The cnidarians which exist in both polyp and medusa forms are known to exhibit metagenesis or alternation of generation, i.e., polyp form produces medusa by asexual means and medusae form the polyps by sexual means.