Which characteristic defines a synchronous motor's operation?

The characteristic that defines a synchronous motor's operation is the ability to maintain constant speed under varying load conditions. Unlike other types of motors, which may experience fluctuations in speed based on the load applied to them, synchronous motors run at a speed that is directly synchronized to the frequency of the power supply; this means they maintain a specific speed regardless of the load variations.

When a synchronous motor is operating, it achieves this constant speed because it is designed to rotate in synchrony with the alternating current (AC) supply frequency. For example, in a system operating at a frequency of 60 Hz, a synchronous motor will have a synchronous speed of 1800 RPM for a four-pole motor. This characteristic is particularly useful in applications where precise speed control is necessary.

While high starting torque and lagging power factor can also be associated with synchronous motors, they do not define the unique operational characteristic of maintaining a constant speed as load changes, which is the primary reason for selecting option B.