The correct answer is D. All of the above.
Satisfactory commutation of D.C. machines requires that the brushes be of proper grade and size, that they smoothly run in the holders, and that the commutator be smooth, concentric, and properly undercut.
The brushes are the electrical contacts that transfer current from the external circuit to the rotating armature of the D.C. machine. The brushes must be of the proper grade and size to ensure good electrical contact and to prevent sparking. The brushes should also smoothly run in the holders to prevent wear and tear.
The commutator is the cylindrical part of the armature that the brushes contact. The commutator must be smooth and concentric to ensure good electrical contact with the brushes. The commutator should also be properly undercut to allow the brushes to make good contact with the commutator segments.
If any of these conditions are not met, it can lead to poor commutation, which can cause sparking, overheating, and damage to the D.C. machine.