The correct answer is: D. All of the above.
Armature reaction is the magnetic field produced by the armature current that opposes the main field of the motor. This can cause a number of problems, including a decrease in motor torque, a decrease in motor speed, and an increase in motor heating.
There are a number of ways to reduce the effects of armature reaction. One way is to increase the length of the air gap at the pole tips. This will reduce the magnetic flux produced by the armature current, and therefore the effect of armature reaction.
Another way to reduce the effects of armature reaction is to increase the reluctance of the pole tips. This can be done by using a material with a higher magnetic permeability, or by using a thicker pole piece.
Finally, compensating windings can be used to reduce the effects of armature reaction. Compensating windings are placed in the stator of the motor, and are designed to produce a magnetic field that opposes the armature reaction field.
All of these methods can be used to reduce the effects of armature reaction, and the best method to use will depend on the specific application.