Introduction:
In polyphase multipolar synchronous motors running at full load, the mechanical displacement of the rotor with respect to the stator is an important parameter. It determines the position of the rotor relative to the stator and affects the performance of the motor. The correct answer to the given question is option 'C', which states that the mechanical displacement is of the order of five degrees.

Explanation:
To understand why the mechanical displacement is approximately five degrees, let's consider some key points about synchronous motors:

1. Synchronous Motors:
Synchronous motors are AC motors that operate at a fixed speed determined by the frequency of the power supply. The rotor rotates at the same speed as the rotating magnetic field produced by the stator.

2. Stator and Rotor Magnetic Fields:
The stator produces a rotating magnetic field, while the rotor contains permanent magnets or electromagnets. The interaction between the stator and rotor magnetic fields results in the rotation of the rotor.

3. Synchronous Speed:
The synchronous speed (Ns) of a synchronous motor is given by the formula:
Ns = 120f / P
where 'f' is the frequency of the power supply in Hz and 'P' is the number of poles.

4. Slip:
The slip (S) of a synchronous motor is defined as the difference between the synchronous speed and the actual rotor speed, expressed as a percentage of the synchronous speed.
S = (Ns - N) / Ns * 100
where 'N' is the actual rotor speed.

5. Full Load Operation:
When a synchronous motor is running at full load, the slip is very small, approaching zero. This means that the rotor speed is very close to the synchronous speed.

Mechanical Displacement:
In a synchronous motor, the mechanical displacement refers to the angular displacement between the rotor and the stator. It is determined by the number of poles and the slip of the motor.

6. Pole Configuration:
The mechanical displacement depends on the pole configuration of the motor. In a polyphase multipolar synchronous motor, the rotor and stator have multiple poles.

7. Order of Mechanical Displacement:
The order of the mechanical displacement can be calculated using the formula:
Mechanical Displacement = 360 / (P / 2)
where 'P' is the number of poles.

In the given question, the motor is described as a polyphase multipolar synchronous motor running at full load. Since the slip is very small at full load, the rotor speed is close to the synchronous speed. Therefore, the mechanical displacement is primarily determined by the pole configuration.

For a polyphase multipolar synchronous motor, the mechanical displacement is generally in the range of 5 to 15 degrees. Option 'C' states that the mechanical displacement is of the order of five degrees, which is a reasonable estimate for such motors.

Conclusion:
In polyphase multipolar synchronous motors running at full load, the mechanical displacement of the rotor with respect to the stator is approximately five degrees. This displacement is primarily determined by the pole configuration of the motor.