Which of the following is NOT a reason for using a stabiliser or variable-incidence tailplane?

The chosen response is correct because the primary function of a stabiliser or variable-incidence tailplane is not to specifically reduce drag during the climb phase of flight. Instead, these components are designed to provide stability and control over a wide range of speeds and configurations.

A stabiliser or variable-incidence tailplane offers balancing forces that help maintain the aircraft's equilibrium, especially across varying speeds, making it essential for handling large trim changes. The design helps mitigate the influence of major aerodynamic shifts that occur when an aircraft changes speed or pitch, hence maintaining control and comfort during flight.

In addition, reducing elevator trim drag is a key advantage of using a stabiliser. By adjusting the angle of the tailplane, pilots can manage the load on the elevators more efficiently, leading to overall reductions in drag when the aircraft is trimmed for different flight conditions.

Thus, while drag reduction can be a benefit in certain scenarios, the specific task of reducing drag during climb is not inherently linked to the purposeful design and function of a stabiliser or variable-incidence tailplane.