Final answer:
The correct phrase describes how the nervous system signals the skeletal system's muscles to contract, creating movement, and how the skeletal system protects components of the nervous system.
Step-by-step explanation:
The phrase that could be placed in the position marked with an X to describe an interaction between the nervous system and the skeletal system is: 'Nervous system controls the movement of the skeleton by signaling muscles to contract'. The nervous system sends electrical signals to the muscles that are attached to the bones of the skeletal system. When muscles receive these signals, they contract, creating movement in the corresponding bones.
Another key interaction between the two systems is the role of the skeletal system in providing protection for the nervous system. For instance, the skull protects the brain, and the vertebrae protect the spinal cord.