The correct answer is: D. Amount of current to be collected.
A commutator is a device used in electric motors and generators to reverse the current in the rotor windings. It consists of a series of copper segments insulated from each other and mounted on a rotating shaft. The segments are connected to the rotor windings, and brushes made of carbon or graphite are pressed against the segments to collect the current.
The number of brushes in a commutator depends on the amount of current to be collected. The more current that needs to be collected, the more brushes are required. This is because the brushes need to be able to carry the current without overheating.
The other options are not correct. The speed of the armature does not affect the number of brushes in a commutator. The type of winding does not affect the number of brushes in a commutator. The voltage does not affect the number of brushes in a commutator.