In D.C. machines, ir order to prevent excessive distortion of field form by the armature reaction, the field mmf must be made

The correct answer is C. large in comparison with the armature mmf.

The armature reaction is the effect of the armature current on the magnetic field produced by the field windings. The armature current produces a magnetic field that opposes the field produced by the field windings. This can cause the field to be distorted, which can affect the performance of the machine.

To prevent excessive distortion of the field, the field mmf must be made large in comparison with the armature mmf. This can be done by increasing the number of turns in the field windings or by increasing the current in the field windings.

Option A is incorrect because if the field mmf is equal to the armature mmf, then the armature reaction will cause the field to be completely distorted. This will have a significant impact on the performance of the machine.

Option B is incorrect because if the field mmf is less than the armature mmf, then the armature reaction will cause the field to be distorted in the opposite direction. This will also have a significant impact on the performance of the machine.

Option D is incorrect because it does not provide a correct answer to the question.