The correct option is D both a and b
Mesoderm is a germinal layer that determines whether an animal is diploblastic or triploblastic. It forms a third germinal layer between ectoderm and endoderm. So, the animals having mesoderm are triploblastic. An animal that does not have mesoderm between the ectoderm and endoderm are diploblastic.
A coelom becomes a true coelom when it is lined by the mesoderm. Animals that have a true coelom are called coelomates. If the mesoderm does not line the body cavity, it is called acoelomates. Therefore mesoderm plays an important role in differentiating between diploblastic and triploblastic animals and coelomate and acoelomate.