The correct answer is A) Variable compressor stator vanes are used. The variable compressor stator vanes in the main propulsion gas turbine are a design feature that helps eliminate compressor surge. By adjusting the angle of the stator vanes, the airflow into the compressor can be optimized to prevent the compressor from entering a surge condition, which can occur at low power settings and lead to engine instability. The other answer choices are incorrect because they do not directly address the design feature used to prevent compressor surge. Maintaining constant turbine inlet temperature (B), constant compressor turbine speed (C), or incorporating a fuel modulating function (D) are not the primary design features used to eliminate compressor surge in a gas turbine engine.