Animals show four different grades of organization:(i) Protoplasmic grade found in protozoans.(ii) Cellular grade is the characteristic of sponges and mesozoans because in a sponge, cells exhibit division of labour for performing specialized functions.(iii) Tissue grade is of two types : Cell-tissue grade is seen in coelenterates as there are cells not only specialised for different functions but also certain similar cells gather together to form tissue as well. Tissue - organ grade appears in flatworms (Platyhelminthes) with arrangement of tissues to form organs.(iv) In organ system level of organisation, organs are joined in a system to perform basic functions. It is found in higher invertebrates and all vertebrates.Hence the correct option is D.