Final answer:
Mammals are divided into three subclasses and about 26 orders. There are approximately 5,400 species of mammals. Traditionally, mammals were classified based on structure and function, but recent DNA analyses have led to the discovery of new relationships among different mammalian species. Option 3.
Step-by-step explanation:
Mammals are divided into three subclasses and about 26 orders. There are approximately 5,400 mammalian species, distributed in 29 orders, 153 families, and about 1,200 genera. Mammals used to be classified based on similarities in structure and function into 17 different orders, but recent DNA analyses have shown that the traditional orders include mammals that are not closely related.
The number of subdivisions in Mammalia is three. These subdivisions are based on reproductive strategies and other distinct characteristics. The three main groups are monotremes, marsupials, and eutherians (or placental mammals). Monotremes are egg-laying mammals, marsupials give birth to live young that usually continue to develop in a pouch, and placental mammals have a complex placenta which allows the offspring to have a long gestation period within the womb. Despite the presence of superorders based on DNA data, the three main subdivisions remain consistent across traditional and modern classifications.