Adding armature windings in parallel to a generator's existing windings:

Adding armature windings in parallel to a generator's existing windings improves its voltage stability. This is because parallel windings can help distribute the load more evenly across the windings, which reduces the impact of fluctuations in the load on the generator's output voltage. When additional windings are added in parallel, they can help maintain a more consistent voltage level by allowing for better regulation of output under varying load conditions.

Furthermore, parallel configurations can contribute to a reduction in resistance and inductive reactance in the circuit, which enhances the overall electrical performance of the generator. This increased capacity to handle transient load changes contributes significantly to the stability of the voltage output, helping to prevent issues such as voltage sag or spikes that could arise during sudden changes in demand.

The other options do not support this improvement in voltage stability, as reducing power output or efficiency would counteract the generator's ability to perform effectively under load. Lastly, stating that there is no effect on the output overlooks the positive implications of parallel configurations on stability and performance.