The correct answer is C) The mica slots are undercut to a depth approximately equal to the width of the slot. The reasoning is that as the brushes ride on the commutator, mica ridges can form between the commutator bars, leading to sparking and brush wear. To address this, the mica is undercut, or removed, to a depth roughly equal to the width of the mica slot. This prevents the mica from protruding above the commutator bars and interfering with the brush contact. The other options are incorrect because A) 1mm is an arbitrary depth unrelated to the width of the mica slot, B) twice the width is excessive and unnecessary, and D) the depth of the undercut is indeed important for proper commutator maintenance.