Correct option is A
Armature reaction: In a DC machine, the current flowing through the armature windings creates its own magnetic field, which interacts with the main magnetic field produced by the field coils. This interaction is called armature reaction.
Cross-magnetization: When the armature current flows, its magnetic field can distort the main field, causing it to be unevenly distributed across the poles. This distortion is referred to as cross-magnetization because the armature field is perpendicular to the main field.
Demagnetization: Demagnetization occurs when the armature reaction actually weakens the main field. This happens when the armature current is in a direction that opposes the main field. While this can happen under certain conditions, in an unsaturated DC machine, the primary effect is usually cross-magnetization.
