The correct answer is option 'B': short-circuited by low resistance.

Explanation:
When starting a synchronous motor, the induction motor action is used to bring the motor up to synchronous speed. In this process, the field winding of the synchronous motor is short-circuited by a low resistance.

Here's a detailed explanation of why the field winding is short-circuited and why it is the correct option:

1. Starting a synchronous motor:
- When starting a synchronous motor, the rotor of the motor is not initially rotating at synchronous speed.
- To bring the rotor up to synchronous speed, an auxiliary motor is used. This auxiliary motor is usually an induction motor.

2. Induction motor action:
- The auxiliary induction motor is connected to the same power supply as the synchronous motor.
- The stator winding of the induction motor is connected to the power supply, and the rotor winding of the induction motor is short-circuited.
- When the power supply is turned on, the induction motor starts rotating.
- As the rotor of the induction motor rotates, it creates a rotating magnetic field.
- This rotating magnetic field induces currents in the short-circuited rotor winding of the synchronous motor.
- These induced currents in the synchronous motor rotor create a magnetic field that interacts with the stator's magnetic field, causing the rotor to start rotating.

3. Field winding of the synchronous motor:
- The field winding of the synchronous motor is responsible for creating the magnetic field that interacts with the stator's field.
- During the starting process, the field winding is short-circuited by a low resistance.
- This short-circuit allows the induced currents from the induction motor to flow through the field winding.
- These induced currents help create the necessary magnetic field in the synchronous motor to start rotating.

4. Advantages of short-circuiting the field winding:
- Short-circuiting the field winding allows for the efficient transfer of induced currents from the induction motor to the synchronous motor.
- It simplifies the starting process and ensures that the synchronous motor reaches synchronous speed.
- It avoids the need for a separate DC supply to power the field winding during starting.

In conclusion, when starting a synchronous motor using induction motor action, the field winding of the synchronous motor is short-circuited by a low resistance. This allows the induced currents from the induction motor to flow through the field winding and create the necessary magnetic field for the synchronous motor to start rotating.