The correct answer is A) dividing the spring force by the brush contact area. The brush pressure for a DC motor is calculated by dividing the spring force that pushes the brush against the commutator by the brush contact area. This ensures the brush maintains adequate pressure to create a good electrical connection and minimize wear. The other answer choices do not correctly describe how to calculate the brush pressure. Subtracting the brush contact area from the spring pressure (B) or dividing the brush contact area by the spring pressure (C) would not result in the proper brush pressure calculation. Subtracting the spring pressure from the brush contact area (D) is also incorrect, as this would not provide the necessary brush pressure.