What best defines the operation of a synchronous motor?

The operation of a synchronous motor is best defined by its ability to start and run on alternating current (AC) while maintaining a constant speed that is synchronized with the frequency of the AC power supply. This means that the motor requires AC for both starting and continuous operation.

In the case of option A, it correctly indicates that the motor starts on AC and continues to operate on AC, aligning with the inherent characteristics of synchronous motors. These motors depend on the AC power supply for their magnetic field to create torque and maintain synchronization with the supply frequency.

Understanding this, it's clear why the other options do not fit. Synchronous motors do not operate on direct current (DC) for their normal function, and they do not start or run from a DC power supply. Therefore, the definition focuses on the utilization of AC for both starting and operation, which is essential for the synchronous motor's functionality.