The correct option is D Scyphozoa
Jellyfishes are marine, free-swimming members of class Scyphozoa with prominent medusa form and reduced or absent polyp stage. Their medusa is comparatively larger than that of hydrozoans and has characteristic thick, viscous mesoglea that imparts firmness to the body. Class Anthozoa includes sessile, marine, polyps, lacking medusa stage. They have gastrovascular cavity divided into chambers to increase the area for digestion. Class hydrozoa includes marine as well as freshwater species that exhibit distinct alternation of polyp and medusa stages. Thus, the correct answer is option B.