What is a defining characteristic of flat bones?

Flat bones are characterized by their thin, flattened shape, which allows them to serve essential functions in the body, particularly concerning muscle attachment and protection of vital organs. The defining characteristic of flat bones, such as the skull, ribs, and sternum, is that they provide a wide area for muscle attachment. This broad surface facilitates the anchoring of muscles, which is critical for movement and stability in the body.

The other options describe features that do not accurately relate to flat bones. Irregular shapes are more typical of bones like vertebrae, while the descriptions of being large and convex or small and cuboid refer to shapes associated with other classifications of bones, such as long bones and short bones respectively. Thus, the ability of flat bones to offer maximum area for muscle attachment is what distinctly defines them in the context of skeletal anatomy.