The correct answer is D. When a DC generator is disassembled during overhaul, it is important to ensure that the brush holder rigging is installed in the neutral plane of the machine. The neutral plane is the position where the armature coils are undergoing zero voltage induced in them, and this typically corresponds to the center of the interpoles on the machine. With the marked armature slots positioned directly under the center of the interpoles, the brushing rigging should be adjusted and clamped so that the brushes are positioned directly over the marked armature bars. This ensures that the brushes are in the neutral plane, which is the optimal position for proper generator operation and commutation. The other options are incorrect because they do not align the brush rigging with the neutral plane as defined by the interpoles. Positioning the brushes over the main field poles, compensating windings, or between the interpoles and main field poles would result in suboptimal generator performance.