Which of the following is NOT one of the major groups of mammals?

The classification of mammals is typically divided into three major groups: monotremes, marsupials, and placental mammals. Monotremes, such as the platypus and echidna, are unique because they lay eggs instead of giving birth to live young. Marsupials, such as kangaroos and koalas, give birth to relatively undeveloped young that often continue to develop in a pouch. Placental mammals, which include a vast majority of mammals like humans and whales, give birth to more fully developed young that have been nourished in the womb via a placenta.

In this context, reptiles are not a group of mammals but rather a separate class of vertebrates that include turtles, snakes, lizards, and crocodilians. They are characterized by features such as scaly skin and the ability to lay eggs (usually), but they do not belong to the mammalian classification. Therefore, identifying reptiles as not being one of the major groups of mammals accurately reflects the taxonomic distinctions between these two classes.