Okay so this is why B is the correct answer.
This is an assumption question. So we want to start by identifying the main conclusion as well as the evidence.
The conclusion is: "So the President's choice was clearly motivated by sound budgetary policy, not partisan politics."
And the evidence is "all of the canceled projects had been identified as wasteful in a report written by respected nonpartisan auditors."
This seems like a good argument. The projects are identified as wasteful and so this is not a partisan issue even though 90% of the canceled projects were in opposition districts.
What you want to look for is a potential problem with this otherwise very sound argument. What could the problem be with the evidence that these projects are identified as wasteful? What if this report identified just about every project in the country as wasteful?? Then the President would be able to choose any project to cancel and say that it was in the report.
So we are looking for the answer choice that protects the argument from this possible flaw. So we want an answer choice that says that it is not true that their were lots of districts from both parties on the list. Because that would mean that the president really was following the report and eliminating wasteful projects.
Choice B is the correct answer. It says "B) The scheduled highway projects identified as wasteful in the report were not mostly projects in districts controlled by the President's party."
Remember, the reason that is given for the President being motivated by budget policy, not partisan politics, is that these projects that are canceled were wasteful so the President just did the right thing. But what if there were lots of projects in districts that favored the President that were also labeled as wasteful and he did not cancel those, but only the ones in opposing districts? That would really weaken this.
Your job is to prevent such a problem. Answer choice B prevents that problem by making it clear that the wasteful projects were not mostly in the district controlled by the President's party.
D has nothing to do with the argument. The argument is talking about waste and not about the expense. A really expensive project may not be wasteful.
I discussed this method of looking at assumption questions in the article https://www.beatthegmat.com/mba/2012/06/ ... -an-expert