The correct answer is: A. The field.
Iron losses are losses in a magnetic circuit due to the hysteresis and eddy current effects in the iron core. The hysteresis loss is caused by the reversal of the magnetic field in the core, while the eddy current loss is caused by the circulating currents induced in the core by the changing magnetic field.
The field is the part of the DC motor that produces the magnetic field that causes the armature to rotate. The field is usually made of a laminated iron core to reduce eddy current losses. The field winding is made of copper wire and is wound around the laminated iron core. The field current is supplied by the DC power supply.
The armature is the part of the DC motor that rotates. The armature is usually made of a laminated iron core with copper windings. The armature windings are connected to the commutator. The commutator is a series of copper segments that are insulated from each other. The commutator is connected to the brushes, which are made of carbon. The brushes are in contact with the commutator and collect the current from the armature windings.
The brushes are not the source of iron losses in a DC motor. The brushes are simply there to collect the current from the armature windings and transfer it to the commutator. The commutator is also not the source of iron losses in a DC motor. The commutator is simply there to provide a means for the brushes to collect the current from the armature windings and transfer it to the external circuit.
The iron losses in a DC motor occur in the field. The field is the part of the DC motor that produces the magnetic field that causes the armature to rotate. The field is usually made of a laminated iron core to reduce eddy current losses. The field winding is made of copper wire and is wound around the laminated iron core. The field current is supplied by the DC power supply.