Hi po1016,
Answer choice (B) indicates a Time Shift error: what has happened in the past may not continue happening in the future, so a nuclear war is still possible. This would be a great answer if the author's argument made any predictions about how nuclear deterrence will work in the future. She does not. The entire argument is about the past: that the policy of deterrence has worked
thus far. The author makes no predictions as to whether it will continue to work in the future, which is why answer choice (B) is irrelevant.
Essentially, the author makes a causal argument: a third world war hasn't happened (yet),
because of the policy of nuclear deterrence:
Nuclear deterrence (cause)
Avoid third world war (effect)
Answer choice (E) raises the possibility of another cause, which is a common way of weakening causal arguments.
Let me know if this helps!