Coelentrates show the phenomenon of polymorphism i.e. occurrence of more than one type of individuals in the same species. The two basic forms of Coelentrates are polyp and medusa.
The polyp form is a sessile, asexually reproducing, nourishment providing colonial form wheras the medusoid form is free swimming, umbrella shaped and sexually reproducing. In members of class Hydrozoa, polyp stage is dominant and medusa stage is reduced or absent. For example, Hydra, Obelia, Physalia. Aurelia (Jellyfish) belongs to Scyphozoan in which medusoid phase is dominant and polyp phase is absent. So, the correct answer is Aurelia.