In a D.C. series motor, if the armature current is reduced by 50%, the torque of the motor will be equal to

The correct answer is: C. 25% of the previous value.

In a D.C. series motor, the torque is proportional to the square of the armature current. Therefore, if the armature current is reduced by 50%, the torque will be reduced by 75%, or 25% of the previous value.

Option A is incorrect because the torque will not be equal to 100% of the previous value. Option B is incorrect because the torque will not be equal to 50% of the previous value. Option D is incorrect because the torque will not be equal to 10% of the previous value.