What must aircraft flight control trim systems allow the pilot to determine?

The correct choice indicates that aircraft flight control trim systems must allow the pilot to determine the position of the trim tab. Trim tabs play a crucial role in maintaining aircraft stability by adjusting the control surfaces, thus enabling the pilot to alleviate control pressures. By being aware of the position of the trim tab, the pilot can make informed decisions about the aircraft's handling characteristics and stability during flight.

Understanding the trim tab's position is essential for the pilot to achieve efficient and safe flight operations. It allows for fine-tuning of control input to counteract any unintentional forces acting on the aircraft, promoting smoother and more efficient flying, particularly during prolonged flight or when adjustments to the aircraft's pitch, roll, or yaw are required.