We're looking for a general principle to support the doctors conclusion that the city should not have repealed the tax after only seven months.
Answer choice C tells us that policymakers should consider the implications of a policy before it goes into effect. This tells us why the law should never have been put into effect in the first place, but it does not directly improve the doctors argument. In fact it might weaken the doctor's conclusion as it implies that the law should have never been passed.
B does give us a reason why the city should not have repealed the tax so soon, as it tells us that regardless of apparent negative effects, we should wait a year to see the full impact. Since the doctor thinks the tax was repealed too soon, B supports their reasoning.