The correct answer is: C. has large values for low and high excitation.
The armature current of a synchronous motor is the current that flows through the armature windings of the motor. The armature current is proportional to the torque produced by the motor. The torque produced by a synchronous motor is proportional to the field current. The field current is the current that flows through the field windings of the motor. The field current is controlled by the excitation voltage.
When the excitation voltage is low, the field current is low. This means that the torque produced by the motor is low. The armature current is also low when the excitation voltage is low. This is because the torque produced by the motor is low, and the motor does not need to draw a lot of current to produce the required torque.
When the excitation voltage is high, the field current is high. This means that the torque produced by the motor is high. The armature current is also high when the excitation voltage is high. This is because the torque produced by the motor is high, and the motor needs to draw a lot of current to produce the required torque.
Therefore, the armature current of a synchronous motor has large values for low and high excitation.