The correct option is D cnidoblast
Presence of cnidoblast or stinging cells is a unique feature of phylum Coelenterata. Cnidoblasts contain stinging capsules or nematocysts. It lines the tentacles which help in anchorage, defence and capturing prey. Due to the presence of cnidoblasts, Coelenterates are also called Cnidaria.
Bioluminescence is the feature of emission of light in living organisms. It is a characteristic feature of phylum Ctenophora.
Flame cells are present in phylum Platyhelminthes. It helps in excretion and osmoregulation.
Hermaphrodites are organisms which have both male and female sex organs in their bodies. It is not a unique feature of Coelenterates. Members of phylum Annelida are also hermaphrodites.