Answer:
D. violence against the Ghost would be useless.
If vulnerare means to wound, then invulnerable means something like not to wound. Marcellus says: "For it is as the air invulnerable."
That means the ghost is like air. You can't hurt air, you can't wound it. So if the ghost is like air, it also cannot be wounded. Trying to hurt the ghost would be useless, because it cannot be hurt. That's why the answer is d.