OA is A.
In an assumption question, the key thing to realize is that you are not looking for the answer choice that does the most for the stimulus when added - you are looking for the answer choice that hurts the most when taken away. The correct answer to an assumption question is something that is "necessary" to the argument. For example, taking the GMAT is necessary for acceptance into most MBA programs. If you take the GMAT away, then the conclusion - being accepted into a MBA program - is harmed. An assumption is not something that has to really strengthen the conclusion when added. For example, just taking the GMAT does not guarantee admission into an MBA program.
In the case of the above problem, the politician concludes that the nuclear power plant should be re-licensed for another 20 years. In other words the activity of producing power at that plant using nuclear fuel should be allowed to continue. The reason given is that, although there have been some problems with the plants operation, none of these safety issues has been conclusively proven to lead to a nuclear disaster.
So there is a gap in the politician's argument that an assumption could fill. The politician jumps from "none of the safety issues has been conclusively proven to lead to a nuclear disaster" to "the plant should be re-licensed for another 20 years." So we can say that the politician believes that ONLY conclusive proof of a negative outcome (nuclear disaster, for example) is sufficient evidence to halt an activity (such as generating nuclear power). This leads directly to answer choice A. And if answer choice A were negated to state that "Evidence that indicates a possible negative outcome IS sufficient to halt the activity that is the source of that outcome EVEN IF the outcome cannot be proven conclusively" this would certainly harm the politician's conclusion. This is called the Assumption Negation Technique in the Veritas Critical Reasoning 2 book.
Answer Choice B is not correct because the argument does not assume that the lack of proof becomes "proof that the negative outcome cannot occur." It is not essential to this argument to show that the negative outcome cannot occur - because the conclusion does not rely on this. The conclusion is only that the power plant should be relicensed, not that the possibility of nuclear disaster no longer exists.
Answer Choice C does not take us to the conclusion of re-licensing the power plant. After all, the stimulus already states that the negative outcome (nuclear disaster) has not been conclusively proven. So answer choice C does not take us any further toward the conclusion.
Answer Choice D states that an activity should be able to continue at a particular location, "unless there is reason to believe that a future disaster...is possible." This particular "unless" is satisfied by the stimulus. There are good reasons to believe that a nuclear disaster at that power plant is very possible - just not 100% proven that it will occur. So, D would actually lead to NO Re-licensing - or the opposite of the politician's argument.
Answer Choice E mentions a "positive outcome that is greater in magnitude than any possible negative outcome..." the argument does not mention any such positive outcome, and for the people of the Northeast U.S. it is hard to imagine the positive outcome that could outweigh a nuclear disaster. So this reasoning would not apply since the conditions are not met.
