The correct answer is B. Compensating windings in DC generators are designed to counteract the armature reaction associated with heavy loads and minimize field distortion. Armature reaction is a phenomenon where the magnetic field created by the armature current interacts with the main field, causing the magnetic field to become distorted. Compensating windings, which are wound on the main field pole pieces, help to offset this effect and maintain a more stable magnetic field, preventing excessive sparking at the commutator. The other options are incorrect because they either describe the purpose of interpoles (which are narrow poles placed in the magnetic neutral plane, not compensating windings), or they do not accurately describe the function of compensating windings in a DC generator.