Final answer:
All mammals are characterized by viviparity, meaning they give birth to live young. The major difference lies in the reproductive methods: monotremes lay eggs, marsupials give birth to underdeveloped young that grow in a pouch, and placental mammals nourish the fetus through a placenta.
Step-by-step explanation:
In the context of reproduction, the characteristic that is common to all mammal species is viviparity, which means they give birth to live young. While most mammals are viviparous, there are a few exceptions. Specifically, monotremes like the platypus and echidnas lay eggs. There are three types of mammals based on their reproductive methods: monotremes, marsupials, and placental mammals. Marsupials, such as kangaroos and koalas, give birth to underdeveloped young which continue to grow in the mother's pouch. The largest group, the placental mammals, which includes humans, dogs, and rabbits among others, nourishes the fetus through a placenta. This group represents the majority of mammalian species.