Solution

The phenomenon of occurrence of more than one type of structurally and functionally different individuals within a population is termed as polymorphism. The class Hydrozoa of phylum Coelenterata includes a large number of colonial species that contain more than one form of individuals which are called zooids. The phenomenon of polymorphism is essentially one of division of labour in which specific functions are assigned to different individuals. Thus, polyps are modified for feeding, protection and asexual reproduction, while medusae are concerned with sexual reproduction.
Therefore, the correct answer is option D.