Which of the following defines a ratite?

A ratite is defined as a flightless bird, which is characterized by the absence of a keel on their breastbone, a feature that enables flight in other bird species. Ratites include such birds as ostriches, emus, kiwis, and cassowaries. They are primarily terrestrial and possess adaptations suited for running rather than flying, which is a crucial evolutionary response to their environment.

The other options do not fit the definition of a ratite. Flying birds, mammals, and reptiles represent entirely different groups within the animal kingdom, each with distinct characteristics that do not overlap with the defining traits of ratites. Therefore, the only accurate answer is that ratites are classified as flightless birds.