The correct answer is: A. high excitation only.
A synchronous motor is an AC electric motor that runs at a constant speed synchronized with the AC power supply frequency. The speed of a synchronous motor is determined by the frequency of the AC power supply and the number of poles on the stator. The armature current of a synchronous motor is proportional to the torque required to drive the load. When the motor is lightly loaded, the armature current is small. When the motor is heavily loaded, the armature current is large.
When the excitation of a synchronous motor is increased, the magnetic field of the stator is increased. This increases the torque that the motor can produce. The increased torque causes the armature current to increase.
When the excitation of a synchronous motor is decreased, the magnetic field of the stator is decreased. This decreases the torque that the motor can produce. The decreased torque causes the armature current to decrease.
Therefore, the armature current of a synchronous motor has large values for high excitation only.