The correct answer is a rollback plan, which is specifically designed to allow an IT system to be returned to a previous, stable state following the unsuccessful application of updates or changes without causing significant service disruption. This is vital, especially for critical systems such as a firewall. A change management policy describes the overall approach and includes the need for such a plan but does not serve as the plan itself. An escalation plan is focused on increasing resources or attention in response to problems rather than reverting changes. Lastly, a contingency plan is a broader approach for any unexpected events and may not be as specific or detailed for reverting a particular change.