Final answer:
Living diapsids include an extremely diverse group of vertebrates: birds and reptiles, which have evolved through the Mesozoic period from the common diapsid lineage.
Step-by-step explanation:
The group of living diapsids is an extremely diverse group of vertebrates which includes birds and various reptiles, such as crocodiles, dinosaurs, and the modern reptiles we see today. Diapsids diverged during the Mesozoic period into Archosauromorpha, which includes modern crocodiles and alligators, and Lepidosauromorpha, which covers modern lizards, snakes, and tuataras. Birds evolved from a branch of maniraptoran theropod dinosaurs and are classified as avian reptiles within Reptilia. Therefore, the correct answer to the student's question is both birds and reptiles.