The correct answer is D) side-cutting the copper segments and undercutting the mica. The proper maintenance of a DC motor's commutator involves periodically side-cutting the copper segments and undercutting the mica between the segments. This helps maintain the proper clearance between the commutator segments and ensures the mica does not become flush with the copper, which can lead to short circuits and motor damage. The other options are incorrect - painting the commutator, baking the armature, or coating with oil are not standard maintenance procedures for a DC motor commutator.