You are correct, the answer is A) sparking of the brushes. Insufficient brush pressure on a DC motor can cause sparking of the brushes. This is because the brushes need to maintain consistent and adequate contact with the commutator in order to properly transfer the electrical current. If the brush pressure is insufficient, the brushes may lose contact with the commutator, leading to arcing and sparking. The other answer choices are incorrect. Excess residual magnetism (B) and water vapor absorption (C) are not directly related to insufficient brush pressure. Generator overload (D) is also not a consequence of this issue.