Most appropriate term for the life cycle of Obelia is

True alternation of generations is alternation between haploid gametophyte and diploid sporophyte phase. In Obelia, the asexual polypoid phase is alternate with sexual medusoid phase and this type of life cycle is called metagenesis.
Neoteny It is also called Juvenilisation, is one of the two ways by which paedomorphism can arise.
Paedomorphism It is the retention by adults of traits previously seen only in young, and is a subject studied in the development biology .
Metamorphosis It is a biological process by which an animal physically develops after birth or notching, involving a conspicous and relatively abrupt change in the animal body structure through cell growth and differentiation e.g., in some insects, molluses etc.