Explanation

For this sentence, it’s important that you know that nuclear holocaust never actually occurred, so it was only a threat that was a daily part of people’s lives.

A secondary meaning of the word specter (which can also mean “ghost”) is “something widely feared as a dangerous occurrence.” All the other words have a kind of categorical relevance, but they don’t fit the sentence.

Specter.
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UPDATED
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The fact that they ran drills shows that they anticipated something terrible and were preparing for it, to be more specific, a nuclear holocaust. Only the word specter which means something widely feared as a possible unpleasant or dangerous occurrence, fits the blank perfectly to convey the meaning that the drills for a preparation to secure themselves as best as possible from a nuclear holocaust.
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