The correct answer is: A. high excitation only.
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. Therefore, the armature current of a synchronous motor is higher for high excitation only.
For low excitation, the field current is low and the torque produced by the motor is low. Therefore, the armature current is also low.
For high excitation, the field current is high and the torque produced by the motor is high. Therefore, the armature current is also high.
The following is a brief explanation of each option:
A. High excitation only: The armature current of a synchronous motor is higher for high excitation only.
B. Low excitation only: The armature current of a synchronous motor is lower for low excitation only.
C. Both (A) and (B): The armature current of a synchronous motor is higher for both high and low excitation.
D. None of the above: The armature current of a synchronous motor is not higher for either high or low excitation.