Protostomia is a clade of animals. Together with the deuterostomes and xenacoelomorpha, its members make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers. The major distinctions between deuterostomes and protostomes are found in embryonic development and are Early in development, deuterostome embryos undergo radial cleavage. So, the correct answer is "Radial cleavage".