Which of the following is true about a synchronous motor?

A synchronous motor operates by having its rotor locked in sync with the rotational speed of the stator's magnetic field. This synchronization is achieved through a process where the rotor is "dragged" into alignment with the rotating magnetic field produced by the stator. The rotor must rotate at the same speed as the magnetic field to maintain this synchronization, hence the motor's name.

This principle is essential for the motor's operation because if the rotor falls out of sync (which can happen if the load increases unexpectedly), the motor will stop functioning effectively. Therefore, option B accurately describes how a synchronous motor achieves its operation.

In contrast, the other options present misinformation about the characteristics of synchronous motors. The rotor does not rotate freely, it must be synchronized with the stator field. Synchronous motors typically operate on AC (alternating current), not DC (direct current), and they rely on a magnetic field that is generated by their stator to function, so stating that they do not require a magnetic field is incorrect. These principles are fundamental to understanding how synchronous motors are designed and operate in various applications.