The correct option is B Platyhelminthes - Acoelomates
Animals possessing coelom are called coelomates. In coelomates, the body cavity is lined by mesoderm. Example - annelids, molluscs, arthropods, echinoderms, hemichordates and chordates.
In some animals, the body cavity is not lined by mesoderm, instead, the mesoderm is present as scattered pouches in between the ectoderm and endoderm. Such a body cavity is called pseudocoelom and the animals possessing them are called pseudocoelomates. Example - aschelminthes.
The animals in which the body cavity is absent are called acoelomates. Example - platyhelminthes.