The correct answer is B) increase or decrease circuit voltage as required. A potential transformer is a device used in electrical circuits to transform or "step down" the voltage to a lower, more manageable level for measurement and control purposes. Its primary function is to safely reduce the high voltage present in electrical systems to a lower voltage that can be easily and accurately measured by instrumentation, such as voltmeters and relays. This allows the circuit to operate at safer voltage levels while still providing the necessary information about the system's performance. The other answer choices are incorrect because a potential transformer does not generate its own electrical power (A), convert AC to DC (C), or transform electrical energy into mechanical energy (D). Its sole purpose is to adjust the voltage levels within the circuit as needed for proper monitoring and control.