The correct answer is A. increase.
The field mmf is the magnetic field produced by the field windings of a DC machine. It is responsible for creating the magnetic flux that induces an emf in the armature windings. The greater the field mmf, the greater the emf and the torque produced by the machine.
However, increasing the field mmf also increases the size and cost of the machine. This is because the field windings must be made of thicker wire to carry the increased current, and the machine must be built to withstand the increased magnetic field strength.
In general, there is an optimum value for the field mmf that will result in the best performance of the machine. Too little field mmf will result in low torque and low efficiency, while too much field mmf will result in high cost and low efficiency.
The choice of field mmf is also affected by the application of the machine. For example, a machine that is used to drive a heavy load will require a higher field mmf than a machine that is used to drive a light load.
In conclusion, the field mmf in a DC machine has a direct impact on the size and cost of the machine. The optimum value for the field mmf is determined by the application of the machine.