A commutator and brushes are typically found in which type of device?

The correct choice is a DC motor. A commutator and brushes are essential components of a DC motor. The commutator is used to reverse the direction of current flow through the armature windings, which allows the motor to produce continuous rotational motion. The brushes, typically made of carbon, maintain contact with the commutator and supply the electric current to the armature.

In a DC motor, this arrangement enables the creation of a magnetic field that interacts with the field magnets, causing the rotor to turn. This setup is pivotal for the operation of DC motors, which are widely used in applications requiring variable speed control and high starting torque.

While other devices like transformers, rotary converters, and induction heaters are important in electrical engineering, they do not utilize commutators and brushes in their operation. A step-down transformer operates based on electromagnetic induction without any moving parts or brushes. A rotary converter can convert AC to DC but typically does so through different means without the classic commutation of a motor. Induction heaters use induction heating principles and do not involve commutation at all. Thus, the presence of a commutator and brushes distinctly identifies a DC motor among the given options.