The provide(s) protection for nerve networks within the nervous system 1.) vertebrae 2.) nerve sheaths 3.) all of the answer choices are correct 4.) skull

Nerve sheaths are tumors so that answer is not correct so you can eliminate that answer.
Because you eliminate that answer you also eliminate all of the answer choices are correct.
Now you have vertebrae and skull left. 
Vertebrae protects the spinal cord. 
The skull protects the nerves also. I would probably pick skull but both skull and vertebrae are correct.

Ansthe eyeball : ) lacrima glands;