The correct answer is C) establishing the copper oxide surface film. Explanation: The formation of a copper oxide surface film on the commutator is an important factor in minimizing DC motor commutator wear. This thin oxide layer helps to reduce friction and wear between the commutator and the brushes, which is crucial for the long-term operation of the DC motor. The oxide film acts as a lubricant, preventing direct metal-to-metal contact and reducing the likelihood of commutator pitting and brush wear. The other answer choices are not as relevant for minimizing commutator wear. Ensuring a low brush current density (A) can help, but the oxide film is the primary factor. Keeping humidity low (B) is not directly related to commutator wear. Excessive brush pressure (D) can actually increase wear and should be avoided.