Why?

Why is D right when there is no mention of unsurpassed military power in the passage?

Even though the phrase "unsurpassed military power" does not appear in the passage, answer choice (D) can be properly inferred from the passage. Let's break it down. Consider hypothetical Nation A. According to the passage, "to maintain military deterrence, [Nation A] would have to be believed [by other nations] to have retaliatory power so great that a potential aggressor nation [let's call it Nation B] would have reason to think it could not defend itself against [Nation A's] retaliation." Assume that Nation A has unsurpassed military power. Doing so is consistent with the facts presented in the passage; nothing that is expressly stated in the passage is undermined or contradicted by this assumption. Now plug these facts into answer choice (D): "It is in the interests of [Nation A, which] seeks deterrence and has unsurpassed military power to let [Nation B] become aware of [Nation A's power of retaliatory attack." This is textually supported: if Nation A wants to maintain military deterrence, it has to make Nation B believe that Nation A has retaliatory power, such that Nation B could not defend itself against Nation A's retaliation. Hope this helps!

Please either diagram this or make a video showing how we should approach and work through this problem. Thanks!

How is this is a "must be true" question? It seems we must make a probable conclusion rather than having definite evidence that's given in the passage.

Never mind, I figured it out!