The correct answer is: D. Shifting of axis of brushes
The self-induced emf in the coil of a DC generator is neutralized by interpoles, dummy coils, and compensating windings. Shifting the axis of brushes does not neutralize the self-induced emf.
Interpoles are small auxiliary poles placed between the main poles of a DC generator. They are connected to the same source of current as the main poles, and they produce a magnetic field that opposes the magnetic field of the main poles. This helps to reduce the sparking at the brushes.
Dummy coils are coils of wire that are placed in parallel with the main coils of the generator. They are not connected to the load, and they do not produce any current. However, they do help to reduce the sparking at the brushes by absorbing some of the self-induced emf.
Compensating windings are coils of wire that are placed around the main poles of the generator. They are connected to the same source of current as the main poles, and they produce a magnetic field that opposes the magnetic field of the main poles. This helps to reduce the sparking at the brushes and to improve the commutation of the generator.
Shifting the axis of brushes does not neutralize the self-induced emf. In fact, it can actually make the sparking worse. This is because the brushes are now in a position where they are more likely to cut across the magnetic field lines. This can cause a sparking arc, which can damage the brushes and the commutator.