The most appropriate term to describe the life cycle of Obelia is:

Explanation

Obelia belong to the Phylum Coelenterata. In Obelia, the lifecycle includes two clearly defined phases i.e a fixed polypoid phase (hydroid colony) and a pelagic medusoid phase. Hydroid colony has no gonads and reproduces by asexual budding to give rise to medusae. On the other hand, medusae reproduce exclusively a by sexual method (ova and sperms) to give rise to new hydroid colonies. This fact apparently seems to have given rise to the idea of alternation of generations, also called metagenesis.