Well, based off the question the man was shot just out side the dura mater which is inside the skull, which is covered with "<span>periosteum". A </span><span>periosteum is covered be fat. In turn the fat is covered by some skin</span>.
But the question asked what's the "inner most layer" it could have penetrated. <span>So the bullet penetrated the inner periosteum.
So your answer would be "</span>d. periosteum" Hope this helps. c: