The correct answer is A) change the number of operating poles. Changing the number of operating poles in a multi-speed AC motor is the most practical method of controlling the RPM while operating at a constant frequency. This is because the RPM of an AC motor is directly proportional to the frequency and inversely proportional to the number of poles. By changing the number of operating poles, the RPM can be adjusted without affecting the input voltage or power factor. The other options, such as changing the input voltage (B) or power factor (C), would not be as practical or efficient for controlling the RPM of a multi-speed AC motor at a constant frequency. Changing the number of brushes (D) is not applicable to AC motors, as they typically do not use brushes.