Based on the vacuities, the reptiles are divided into four subclass as anapsida, parapsida, diapsida and synapsida.
Anapsida includes amniotes whose skull does not have openings near the temples. They includes turtles and all their extinct relatives. Parapsida includes organisms or reptiles in which the skull has two dorsal temporal openings which includes the ichthyosaurs and related extinct forms. Diapsida includes the group of amniote tetrapods that developed two holes in each side of their skull. They include crocodiles, lizards, snakes, tuatara, birds and nonavian dinosaurs. Synapsida includes mammals and every animal more closely related to mammals than to other living amniotes. They were the largest terrestrial vertebrates in the Permian period 299 to 251 MYA. Therefore, the correct answer is option A.