An aircraft about to take off must leave clear on its left any aircraft that has what status?

The correct choice indicates that an aircraft about to take off must leave clear on its left any aircraft that has taken off. This is rooted in the principle of air traffic management, which prioritizes maintaining safe distances between aircraft during various phases of flight.

When an aircraft has taken off, it is already in the air and may be on its way to climbing or maneuvering in the traffic pattern. Therefore, it is essential for the departing aircraft to ensure that it does not interfere with the flight path of the airborne aircraft to maintain safety and avoid any potential collision.

In contrast, the other statuses listed pertain to different phases of aircraft operation where the rules governing spacing and separation differ. An aircraft about to land is typically managed by air traffic control protocols rather than being of immediate concern for an aircraft preparing for takeoff. An aircraft that is taking off may still be in the process of gaining altitude, and one that is taxiing is still on the ground and would not have the same separation requirement. Thus, the necessity to leave a clear space on the left is specifically related to ensuring safety for those already in the air.