In cruise flight, having an aft centre of gravity will result in what effect on longitudinal static stability?

An aft center of gravity (CG) in an aircraft during cruise flight negatively impacts longitudinal static stability, leading to decreased stability. Longitudinal static stability refers to the aircraft's tendency to return to equilibrium after being disturbed around the longitudinal axis (pitch).

When the CG shifts rearward—toward the tail—it can cause the aircraft to become less stable. This occurs because the moments generated by any disturbance (such as turbulence or an abrupt control input) become less effective in returning the aircraft to its original pitch attitude. A rearward CG increases the risk of the aircraft pitching up excessively or losing control during pitch inputs due to this diminished restoring moment.

In a stable configuration, a forward CG provides a greater moment arm necessary to pitch the nose back down when it’s disturbed, thereby enhancing stability. As stability decreases, the pilot may notice that the aircraft requires more frequent and larger control inputs to maintain desired flight attitudes.

The other options either imply that an aft CG has no effect or suggests that it enhances stability, which are inaccurate concepts in the context of longitudinal static stability. Thus, having an aft CG diminishes the aircraft's ability to maintain stable flight attitudes effectively.