Final answer:
Option c, 'Red bone marrow is the site of hematopoiesis,' is the correct answer, as red marrow creates new blood cells and is located in the spongy bone of long bones and the diploë of flat bones.
Step-by-step explanation:
The correct answer to the question "Which statement is true regarding the skeletal system?" is option c: Red bone marrow is the site of hematopoiesis.
Hematopoiesis refers to the process of creating new blood cells and occurs in the red marrow, which is found in the trabecular cavities of spongy bone of long bones, and in the diploë (spongy bone) of flat bones.
Regarding the other options: a) Flat bones are not composed completely of compact bone, they have a layer of spongy bone sandwiched between layers of compact bone; b) External stresses, such as exercise, generally increase bone matrix density, not decrease it, due to the bone adapting to the load by remodeling; d) Osteoclasts are actually responsible for bone resorption, not deposition, so they do not secrete collagen fibers to increase bone density, that role is performed by osteoblasts.