The correct answer is: C. field current and speed.
A V-curve is a graph that shows the relationship between the field current and the speed of a synchronous motor. The V-curve is typically shaped like a V, with the field current on the vertical axis and the speed on the horizontal axis. The V-curve is used to determine the operating point of a synchronous motor. The operating point is the point on the V-curve where the field current and the speed are equal. The operating point is important because it determines the power factor and the efficiency of the synchronous motor.
The armature current is the current that flows through the armature winding of a synchronous motor. The armature current is not related to the speed of the synchronous motor. The power factor is the ratio of the real power to the apparent power. The power factor is not related to the speed of the synchronous motor.
The field current is the current that flows through the field winding of a synchronous motor. The field current is related to the speed of the synchronous motor. The higher the field current, the higher the speed of the synchronous motor.