The correct answer is B) directly increase the velocity of the liquid being pumped. The basic function of a centrifugal pump impeller is to impart kinetic energy to the liquid flowing through the pump. As the impeller rotates, it creates a centrifugal force that accelerates the liquid, directly increasing its velocity. This increase in velocity results in a corresponding increase in the pressure of the liquid, as described by Bernoulli's principle. The other options are incorrect because: A) the impeller does not separate air from the liquid, that is the function of other pump components; C) the impeller directly increases the velocity, not the pressure, of the liquid; and D) the impeller converts the kinetic energy of the liquid, not its potential energy, to a higher pressure.