The correct answer is option 'D' - Any of the options.

A synchronous motor is a type of AC motor that operates at a constant speed, determined by the frequency of the power supply. It is designed to run in synchronism with the rotating magnetic field produced by the stator. However, there are several factors that can prevent a synchronous motor from successfully pulling into synchronism. These factors include:

a) Excessive load: If the motor is subjected to an excessive load, it may not have enough torque to overcome the load and achieve synchronism. The motor will continue to run at a slightly slower speed than the synchronous speed, resulting in a slip between the rotor and the rotating magnetic field. This slip causes the motor to lose synchronism.

b) Low excitation: The excitation current is used to create the magnetic field in the rotor of the synchronous motor. If the excitation current is too low, the rotor magnetic field may not be strong enough to lock in synchronism with the rotating magnetic field of the stator. As a result, the motor will fail to pull into synchronism.

c) High friction: Friction in the motor's bearings or other mechanical components can create additional resistance that the motor needs to overcome in order to achieve synchronism. If the friction is too high, the motor may not be able to reach the synchronous speed and will fail to pull into synchronism.

d) Any of the options: It is also possible for a synchronous motor to fail to pull into synchronism due to a combination of factors, such as excessive load, low excitation, and high friction. These factors can act together to prevent the motor from achieving synchronism.

In conclusion, a synchronous motor may fail to pull into synchronism due to excessive load, low excitation, high friction, or a combination of these factors. It is important to ensure that the motor is properly sized, the excitation current is at the appropriate level, and the mechanical components are in good working condition to prevent synchronization issues.