The animal with bilateral symmetry in young stage and radial pentamerous symmetry in the adult stage belongs to the phylum.

The correct option is C Echinodermata

Echinoderms are exclusively marine invertebrate, having some interesting features. Their life cycle is usually complex. In most echinoderms, the embryo develops into a planktonic larva with a bilateral symmetry, that attains a pentaradial symmetry in the adult stage.

Example, Asterias

So, the correct answer is option D.