Why not A?

This is a classic scenario where we are ignoring the middle ground. We are told that if workers do not find their assignments challenging, they become bored and achieve less, but when they find their assignments too challenging, they give up and achieve less than what they are capable of. From this we conclude that no worker's full potential will ever be realized. But, couldn't workers be PERFECTLY challenged? Couldn't there be a scenario where a worker is just challenged enough where they aren't bored and they don't want to give up? We are ignoring this situation and believing that there are only two possibilities, when there could be a lot of areas in between "not challenged" and "too challenged." This is exactly what answer choice (B) is describing: "assuming without warrant that a situation allows only two possibilities." Answer choice (A) is not what the argument is doing. The actual versus the merely possible is not discussed in the argument. It is merely stated that workers can either be "not challenged at all" or "overly challenged." We are not equating actuality with possibility. Thus, answer choice (B) is the correct answer. Hope that clears things up! Please let us know if you have any other questions.

Can the rest of the answer choices be explained as well?

Hi @NS1, Happy to help! Answer choice (C) is incorrect because this is not a flaw in the passage's reasoning. It is perfectly acceptable to rely on subjective evidence in some cases where the conclusion is not dependent upon precise measurements or numbers. It makes sense to use subjective evidence here because the conclusion is based on the workers' experiences. Answer choice (D) is incorrect because neither the conclusion nor the premises are causal. They are correlative - remember that correlation does not equal causation! Answer choice (E) is incorrect because there is no term that is used ambiguously. The passage concludes that no worker will ever realize their full potential based on the fact that assignments that are either not challenging enough or too challenging result in workers achieving less than they are capable of (i.e. less than their full potential). Hope this helps! Please let us know if you have any further questions.