Correct Answer: D
Explanation:
(D) To be impregnable is to be able to withstand almost any kind of assault; to be immutable is to be unchangeable, and therefore able to turn away virtually any kind of alteration.
Immutable: never changing or varying; unchangeable
Alteration: the result of this; change
Intractable: not tractable; specif., a) hard to manage; unruly or stubborn b) hard to work, manipulate, cure, treat, etc.
Destruction: the cause or means of destroying