The correct answer is: C. unity or 0.8 leading.
A synchronous motor is an AC electric motor that converts electrical energy into mechanical energy. It has a rotating magnetic field that is synchronized with the stator winding current. The synchronous motor is a constant-speed motor, meaning that its speed is directly proportional to the frequency of the AC power supply.
The power factor of a synchronous motor is the ratio of the real power (the power that is used to do work) to the apparent power (the product of the voltage and current). The power factor of a synchronous motor is typically between 0.8 and 1.0. A power factor of 1.0 indicates that all of the electrical energy is being used to do work, while a power factor of 0.8 indicates that 20% of the electrical energy is being wasted as heat.
The standard full-load power factor ratings for synchronous motors are unity or 0.8 leading. A power factor of unity indicates that all of the electrical energy is being used to do work, while a power factor of 0.8 leading indicates that 20% of the electrical energy is being used to create a magnetic field that helps the motor to rotate.
Option A is incorrect because a power factor of zero indicates that no electrical energy is being used to do work. Option B is incorrect because a power factor of 0.8 lagging indicates that 20% of the electrical energy is being wasted as heat. Option D is incorrect because a power factor of unity or zero is not a standard full-load power factor rating for synchronous motors.