This is an argument known as a false dilemma. There is a short wikipedia article on it here:
http://en.wikipedia.org/wiki/False_dilemmaAnyways, you will often hear people that are trying to convince you to take a course of action (*cough* politicians *cough* *ahem*) that there are 2 courses of action you can take to produce a desired result and failure to choose the course of action that they suggest will result in dire consequences. Always be suspicious of this. A proper way to respond in thinking like a debater would be "Are those
really the only 2 options? Might there be other options to achieve the desired result that we have not yet considered?" That is what answer choice B is getting at. The commissioner presents 2 options to achieve the desired results, states that 1 of them is impossible, and as a result concludes that the only way to achieve the desired result is to adopt his plan.
When I was reading this I thought "Maybe someone else has come up with a budget that cuts different expenditures then the plan that you have proposed so in that case there would be more then 1 way to achieve the desired result." That is what answer choice B is getting at. It basically says "confuses the fact that while the commissioners plan is sufficient to achieve the desired result we can't be sure it is absolutely
necessary or
required (these terms are interchangeable) to achieve the desired result." There could be other plans that could also achieve the same result in a different way.
Let me know if you need any further clarification on anything I've presented.