The internal feature of bone that makes it simultaneously strong and lightweight is the

The correct answer is the orientation of trabeculae in the spongy bone.  

In the long bones like the femur, at the terminals, the bone is not solid but it comprises holes, which are associated with each other by thin rods and plates of bone tissue called trabeculae.  

This kind of porous or spongy bone is called cancellous bone and is observed at the terminals of the long bones like the femur, and also in the ribs, pelvic bones, vertebrae in the spinal column, and the skull.  

Though cancellous bone comprises a lot of holes in it, it is strong and robust. Each time an individual run, jump, or walk, he or she implies mechanical stresses on the bone in the spine, hips, and pelvis, and they don't break. This strength and support are given by the trabeculae. It gives both strength and also lightweight to the bone due to its porous nature.