OA: B

Fact: Press Secretary: Our critics claim that the President's recent highway project cancellations demonstrate a vindictive desire to punish legislative districts controlled by opposition parties.

Fact: They offer as evidence the fact that 90 percent of the projects canceled were in such districts. But all of the canceled projects had been identified as wasteful in a report written by respected nonpartisan auditors.

Conclusion: So the President's choice was clearly motivated by sound budgetary policy, not partisan politics.

B. The scheduled highway projects identified as wasteful in the report were not mostly projects in districts controlled by the President's party.

Negating B =
B. The scheduled highway projects identified as wasteful in the report were mostly projects in districts controlled by the President's party.

Conclusion: So the President's choice was clearly motivated by sound budgetary policy, not partisan politics.

Option B when negated breaks the conclusion.
Reasoning: If most of the Projects that were identified as wasteful were in presidents parties district then in that 90% project cancelled in the districts that are not controlled by the presidents party indicated the clear bias. the conclusion cant stand with the negated version. Therefore, Option B is the unstated assumption without which the arguments can't stand.

Question #1 - Is my reasoning correct?

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Question #2 - I also though on the line of Prethinking. If we are following this logic then we even do not need to consider Option E as a contender.
Reasoning: The correct option must be able to add a missing premise that is able to fill the missing information that what was the %age of wasteful projects in the districts that are not controlled by President and his party. if this was high then that would confirm that there is no bias.
Right? Option E is unable to answer this question.


Question #3 -
There is one more layer of understanding, although that is not taken care either in the argument or in the answer choices. I will try to explain it with the help of an example-

Let us take an example -
Total Projects =100
Projects in the districts that are not controlled by the Presidents Party = 30
Projects in the districts that are controlled by the Presidents Party = 70

Total Projects identified as wasteful = 30
Out of these 30 -
27 were in districts that are not controlled by the presidents party = 90%
03 were in districts that are controlled by the presidents party = 4.2%

Clearly the percentage is so low in the districts that are controlled by the Presidents Party.
Here there can be one more assumption that Presidents party didn't influenced the decision while declaring which project is wasteful. In short President didn't improvise and influenced so that total number of wasteful projects in the districts of his government are very insignificant.

Although such option is not given, but Isn't my point and pre-thinking valid?