The correct answer is C) misalignment of the brush rigging. Misalignment of the brush rigging can cause brush sparking in a DC generator during commutation. When the brush rigging is not properly aligned, the brushes may not make full contact with the commutator, leading to uneven current distribution and increased resistance at the brush-commutator interface. This can result in arcing and sparking between the brushes and the commutator, which is known as brush sparking. The other options are incorrect because: A) Eddy currents in the armature core can cause heating and efficiency losses, but do not directly contribute to brush sparking. B) Excessive brush pressure can cause excessive wear on the brushes and commutator, but does not necessarily lead to brush sparking. D) Hysteresis in the armature core can affect the magnetic field, but does not directly impact the commutation process and brush sparking.