The correct answer is B) To impart acceleration to the air mass, resulting in an increase in velocity. The primary function of an axial compressor rotor blade is to accelerate the air flow, increasing the velocity of the air stream. As the air passes over the curved shape of the rotor blade, the blade imparts kinetic energy to the air, causing it to speed up. This increase in velocity is a key part of the compression process in an axial compressor. The other answer choices are incorrect because they do not accurately describe the primary function of the rotor blade. Option A is incorrect as the rotor blades do not rely on centrifugal force to increase pressure. Option C is incorrect as the rotor blades do not act as a diffuser to increase pressure. Option D is incorrect as the primary function is to accelerate the air, not change its direction.