The correct answer is B) hysteresis loss. Hysteresis loss is the energy dissipated in the magnetic material of the armature core due to the reversals of magnetization lagging behind the current reversals. This produces heat in the armature core. The use of soft silicon steel minimizes this hysteresis loss by reducing the energy required to reverse the magnetic domains within the core material. The other options are incorrect because: A) Eddy-current loss is caused by induced currents circulating within the armature core, not due to the lag in magnetization reversals. C) Capacitive reaction is related to the capacitive behavior of electrical circuits, not to the magnetic properties of the armature core. D) Copper loss is the energy dissipated in the armature windings due to the electrical resistance of the copper conductors, not related to the magnetic properties of the core.