The correct answer is: B. equal to
A D.C. machine has a number of poles that are equal to the number of brush arms. The poles are the parts of the machine that create the magnetic field, and the brush arms are the parts that conduct the current to the rotor. The number of poles and brush arms must be equal in order for the machine to work properly.
Option A is incorrect because the number of brush arms cannot be less than the number of poles. If there are fewer brush arms than poles, then not all of the poles will be able to conduct current, and the machine will not work properly.
Option C is incorrect because the number of brush arms cannot be greater than the number of poles. If there are more brush arms than poles, then some of the brush arms will be redundant, and the machine will not work any better than if it had the same number of brush arms as poles.
Option D is incorrect because the number of brush arms must be equal to the number of poles.