The correct answer is: C. Copper loss.
Copper loss is the heat loss that occurs in the windings of a synchronous motor due to the resistance of the copper wire. The amount of copper loss is proportional to the square of the current, which means that it increases with the load on the motor.
Windage loss is the loss of energy due to the friction of the air against the rotating parts of the motor. This loss is relatively small and is not affected by the load on the motor.
Bearing friction loss is the loss of energy due to the friction of the bearings against the rotating shaft of the motor. This loss is also relatively small and is not affected by the load on the motor.
Core loss is the loss of energy due to the eddy currents that are induced in the magnetic core of the motor. This loss is proportional to the square of the flux density, which means that it increases with the load on the motor. However, the core loss is usually much smaller than the copper loss, so it does not have a significant effect on the overall efficiency of the motor.
In conclusion, the copper loss is the only loss that varies with load in a synchronous motor.