The correct answer is: C. Either of (A) and (B) above.
A DC series motor is a type of electric motor that uses a DC power source to create a magnetic field that rotates the motor’s armature. The direction of rotation of a DC series motor can be changed by interchanging the supply terminals or the field terminals.
When the supply terminals are interchanged, the direction of the current through the armature is reversed. This reverses the magnetic field created by the armature, which causes the motor to rotate in the opposite direction.
When the field terminals are interchanged, the direction of the magnetic field created by the field coils is reversed. This reverses the magnetic field that interacts with the armature, which causes the motor to rotate in the opposite direction.
Therefore, either interchanging the supply terminals or the field terminals will change the direction of rotation of a DC series motor.