A DC generator produces direct current through electromagnetic induction. The armature windings rotate within the magnetic field created by the field coils. As each conductor cuts through the magnetic flux, an EMF is induced. Because the conductor alternates sides of the field every half revolution, the raw armature output is inherently AC.

To deliver DC to the external circuit, this AC output must be rectified mechanically. The commutator is the mechanical rectifier that performs this conversion. It consists of copper segments mounted on the armature shaft, insulated from each other, with carbon brushes pressing against them. The segments are connected to the armature windings such that each brush always contacts the segment producing current in the correct direction. This switching action converts the alternating armature output into unidirectional (DC) current at the output terminals.

A commutator → CORRECT. The commutator is the mechanical rectifier of the DC generator. It consists of segmented copper bars on the armature shaft connected to the armature windings, with carbon brushes contacting the rotating segments. As the armature turns, the commutator switches the external circuit connection between windings such that the current always flows in the same direction through the external circuit, converting the internally generated AC into DC output. 

An armature → INCORRECT. The armature is the rotating winding assembly of the DC generator in which the EMF is induced by rotation within the magnetic field. The armature generates the AC internally but is not itself the conversion device. 

A transformer → INCORRECT. A transformer is a static electromagnetic device used to change voltage and current levels in AC circuits.

Slip rings → INCORRECT. Slip rings are continuous circular conductors mounted on the rotor shaft that allow a continuous electrical connection between the rotating shaft and the external circuit through brushes. Slip rings do not switch or rectify the current and therefore transmit the AC output of the rotor directly to the external circuit without any modification.