joshnsit
Isn't B a simple reformat of conclusion of A? Can an assumption be same as conslusion in an argument?
I don't think you've correctly identified the conclusion.
First, in any of these 'policy proposal' questions, it is absolutely crucial to identify the *goal* of the policy. Here, a proposal is made so that the bank can 'more effectively shape global economic policy.' The conclusion of the argument is that voting shares should be changed. The assumption then is that voting shares actually matter; they affect how successfully the bank shapes economic policy, which is why B is the correct answer.
To give a simpler example, if I say 'in order to improve your GMAT score, you should do practice problems', I'm assuming that doing practice problems actually has an effect on your GMAT score.