Which of the following assists in the reduction of sparking at the brushes?

The correct choice, which pertains to the positioning of the neutral axis, is vital in reducing sparking at the brushes in electric machines, particularly in DC motors and generators. The neutral axis is the position where the armature windings are not carrying any current. When the armature is aligned correctly with the neutral axis, the brushes make contact with the commutator segments when there is minimal current flowing through the windings. This alignment minimizes the likelihood of sparking because the brushes only encounter segments at momentary intervals when there is no significant voltage difference, thus reducing wear and extending the lifespan of the brushes.

The other options do not effectively address the issue of sparking at the brushes. Increased armature speed can actually lead to more significant sparking due to higher centrifugal forces and increased electrical load on the brushes. A larger air gap typically reduces magnetic efficiency and may also exacerbate sparking issues by increasing the reactive forces involved. Lowering the voltage supply does not specifically target the issue of sparking at the brushes, as it can affect performance and overall system efficiency. Therefore, positioning the neutral axis correctly is the most effective method for minimizing sparking at the brushes.