What action is necessary for the successful completion of a turn?

Maintaining the required bank angle is crucial for the successful completion of a turn because it directly influences the aircraft's rate of turn and its overall control during the maneuver. A proper bank angle allows for a coordinated turn, where the aircraft can effectively balance the horizontal lift component against the vertical component to maintain altitude. If the bank angle is too shallow, the aircraft will not turn efficiently, potentially leading to a wider turn radius. Conversely, if the bank angle is too steep, it might induce excessive load factor, making it more likely for the aircraft to stall.

The other choices do not directly pertain to the fundamental principles of executing a turn. Although increasing speed can affect performance, it is not an action required for a successful turn in the immediate sense. Disengaging the autopilot may be necessary for certain aircraft or circumstances, but it is not universally required. Adjusting the trim system can help alleviate control pressure during prolonged flight, but it does not directly impact the fundamental mechanics of executing a turn.