Please explain

Here we have a classic causation/correlation flaw. Conclusion: the policy of nuclear deterrence has worked thus far. Why? "The proof is that a Third World War between superpowers has not happened." So there is a correlation with the fact the there is a policy of nuclear deterrence and we know that the Third World War has not happened. But, just because those two things are correlated, does not mean that the policy has caused there to be no Third World War. It is very possibly that a different factor is the cause of there being no Third World War. This is exactly what answer choice (E) explains: "it cannot be known whether it was nuclear deterrence that worked, or some other factor, such as recognition of the economic value of remaining at peace." Answer choice (E) explains that there could be an unrelated third factor that has caused the absence of a Third World War, i.e. It clearly delineates the flaw in the issue: correlation/causation flaw. Hope that was helpful! Please let us know if you have any other questions.

Thank you.

I read the conclusion to read "The policy of nuclear deterrence has worked thus far IS QUESTIONABLE" So why would that make answer choice E correct since is that would almost directly invalidate the conclusion?

@ddupray it looks like you simply misread that first sentence. It reads "That the policy of nuclear deterrence has worked thus far is UNquesitonable." In other words, NOT questionable.