The correct answer is A) heat generated by magnetic polarity reversals. Hysteresis is the energy loss that occurs due to the changing magnetic polarity within the core of an electrical generator or transformer. As the magnetic field rapidly reverses direction, the core material experiences energy losses due to the realignment of the magnetic domains, which manifests as heat generation. This heat loss is the phenomenon that results in hysteresis. The other options are incorrect because they do not directly describe the underlying cause of hysteresis. Excessive field current, pulsating terminal current, and arcing at the brushes can all contribute to power losses, but they are not the primary cause of hysteresis.