Which of the following statements is true regarding the structure of flat bones?

The statement that a layer of spongy bone is found within flat bones is true. Flat bones, such as those in the skull, ribs, and pelvis, have a structure that includes an outer layer of compact bone surrounding an inner layer of spongy bone, also known as trabecular bone. This spongy bone contains red bone marrow, which plays a crucial role in hematopoiesis (the formation of blood cells). The presence of spongy bone enhances the strength and flexibility of flat bones while reducing their overall weight.

The other statements do not accurately describe the structure of flat bones. They do have marrow, specifically red bone marrow within the spongy bone. Flat bones are also not made entirely of compact bone; their unique combination of both compact and spongy bone contributes to their functionality and integrity.