The correct answer is D.
Pole shoes are the parts of a DC machine that direct the magnetic field produced by the field coils. They are located at the ends of the stator and have a curved shape that helps to concentrate the magnetic field. This concentration of the magnetic field creates a stronger magnetic field, which in turn creates a stronger torque on the rotor.
The pole shoes also help to spread out the flux to achieve uniform flux density. This is important because it ensures that the rotor is evenly magnetized, which results in a smoother rotation.
Finally, the pole shoes support the field coils. The field coils are the coils of wire that produce the magnetic field. The pole shoes provide a surface for the field coils to be mounted on, and they also help to keep the field coils in place.
In conclusion, the pole shoes in a DC machine serve three important functions: they direct the magnetic field, they spread out the flux to achieve uniform flux density, and they support the field coils.