Which type of airspeed is used in flight planning and is relative to undisturbed air mass?

True airspeed is the correct answer because it represents an aircraft's speed relative to the surrounding undisturbed air mass, which is crucial for flight planning. This measure takes into account the effects of altitude and temperature, providing a more accurate assessment of performance, fuel consumption, and expected flight time compared to indicated airspeed, which is the speed shown on the aircraft's instrument and can be affected by atmospheric conditions and instrument errors.

While ground speed is the speed of the aircraft relative to the ground, it does not provide relevant information about the aircraft's performance in relation to the air through which it is flying. Indicated airspeed, although essential for operational limits, is not a true representation of the aircraft's performance relative to the air mass. Pressure altitude relates to the altitude of the aircraft based on atmospheric pressure and does not directly indicate speed. Thus, true airspeed is the vital parameter for effective flight planning and navigation in relation to the air.