What does a stabiliser/variable-incidence tailplane do?

A stabiliser or variable-incidence tailplane plays a crucial role in the aircraft's aerodynamic stability and control, particularly in maintaining proper longitudinal balance. By adjusting the angle of the tailplane, this component can optimize the lift produced by the tail section of the aircraft, thereby affecting the overall attitude and stability during different phases of flight, including cruising and maneuvering.

The ability to change the angle of attack of the stabiliser allows pilots to fine-tune the aircraft's balance without the need for excessive control inputs, which is particularly important during various flight conditions. This adjustment helps prevent issues such as stalling by ensuring that the aircraft remains within its optimal performance envelope.

In contrast, the other options do not accurately represent the function of a stabiliser/variable-incidence tailplane. It is not limited to providing lift only during takeoff, nor is it used exclusively for landing purposes. Additionally, a variable-incidence tailplane is not a fixed control surface; its primary characteristic is its capability to change its angle, which is fundamental to its function in enhancing the aircraft's stability and control.