The correct answer is C) directly increase the velocity of the liquid being pumped. The basic function of the centrifugal pump impeller is to convert the mechanical energy from the rotating impeller into kinetic energy in the liquid being pumped. As the impeller spins, it creates a pressure difference that draws the liquid into the center of the impeller and then flings it outward, directly increasing the velocity of the liquid. This increase in velocity is the primary function of the centrifugal pump impeller. The other answer choices are incorrect because: A) Separating air from the liquid is not the primary function of the impeller, B) Converting potential energy to kinetic energy is a secondary effect, not the basic function, and D) Directly increasing pressure is also a secondary effect, not the primary function.