The stopway is an area which allows an increase only in the:

A stopway is an area at the end of the runway that is specially designed to provide an additional distance for an aircraft that needs to stop during takeoff. This area is not available for takeoff; rather, its primary purpose is to enhance safety by decreasing the risk associated with an overrun of the runway.

When considering its effect on performance calculations, the stopway is specifically relevant to the accelerate-stop distance available, which is the distance required for an aircraft to accelerate to a predetermined speed and then come to a complete stop in the event of an aborted takeoff. The inclusion of a stopway increases this distance, allowing pilots to have a safer margin should they need to reject the takeoff due to an emergency.

Therefore, the correct answer reflects how the presence of a stopway contributes to allowing a greater margin for stopping distance, while the other options pertain to distances that are not directly impacted by the existence of a stopway.