The correct answer is D) misalignment of the brush rigging. Brush sparking in a DC generator during commutation is typically caused by improper alignment of the brush rigging. When the brush rigging is misaligned, the brushes may not make proper contact with the commutator segments, leading to arcing and sparking. This can disrupt the commutation process and cause issues with the generator's performance. The other options are not the primary causes of brush sparking. Eddy currents and hysteresis in the armature core can contribute to power losses, but they do not directly cause brush sparking. Excessive brush pressure can also lead to issues, but it is not the main factor responsible for brush sparking during commutation.