The correct answer is
A policy, implemented some time ago, can exist already. The zero-alternative means that the business will be as usual; there will be no change in policy.
A scenario describes uncertain developments outside the scope of the policy makers. Examples: the amount of sea level rise, macro economic development and international relations (war).
It is of no use to describe and evaluate alternatives which have no chance to survive in the decision making process. You should try to describe only realistic alternatives.