Hi Nishant -
I think this is just a poorly worded question. You are correct that if it had asked about the cost baseline, that you would be able to rule out option C, which references (albeit with confusing wording) the management reserves. Since it asked about the budget, however, both A and C would seem to be technically correct.
Based on the answer feedback, it would appear that whoever wrote the question is indeed confusing budget with baseline. So we can write this one off as a bad question. (And though I am not familiar with the book you referenced, I would have doubts about the quality of the information presented based on what you have shared.)
That being said, if this were a real exam question that you ran across and you had two answers that seemed to be correct, this is when you have to go with the "best option available" approach. In this case, answer choice A is clearly correct, since this is included in both the budget and cost baseline, whereas choice C is only correct in one of those two interpretations. So your best bet would be to go with A, since that is probably what the question author had in mind. (In practice, though, I doubt this particular question would have made it through the review process and onto the actual exam.)