Press Secretary: Our critics claim that the Presidents recent highway

Pre-thinking an assumption can really help you stay on track and identify the correct answer quickly. There may be multiple assumptions but pre-thinking is useful in most cases because you understand the argument well before jumping into the options.

Argument:
There are districts controlled and the President and there are some controlled by the opposition parties. The President canceled some projects. 90% of those were located in the opposition party districts. So opposition has been crying foul. The secretary is defending the President. He says that all of these were identified as wasteful by non partisan auditors.
Conclusion: the President's choice was clearly motivated by sound budgetary policy, not partisan politics.

Now we have to think an assumption for this conclusion.

Think of a political argument in which you are taking part. You have to assume that whatever the other person says is the truth. You have to put forward a counter point keeping that in mind. So the other person says, 'all of these were identified as wasteful by non partisan auditors.'
The question that should come to your mind is: which other projects did the non partisan auditors identify as wasteful? Say, they identified 20 wasteful projects. 12 from the President's districts and 8 from the opposition's. What if the President chose all the projects to be canceled from the 8 wasteful projects of the opposition's districts? Everything said in the argument stays true but the conclusion becomes invalid. The President would have been motivated by partisan politics in that case.
The assumption you are looking for: Not many of the projects identified as wasteful were from the President's districts.

Hence (B) is your assumption.