Which of the following is a braking method used for a motor?

The choice that reflects a valid braking method for a motor is the option that states "Any of these." Each of the listed methods — friction, plugging, and dynamic braking — are recognized techniques utilized to slow down or stop a motor's operation.

Friction braking involves applying a mechanical force to create resistance against the motor's movement, thereby dissipating kinetic energy as heat. It is commonly seen in applications such as electric vehicles or machinery where additional braking forces may be needed.

Plugging refers to the method of reversing the motor's supply of power momentarily, effectively forcing the motor to slow down quickly. This method is especially useful in applications where rapid stopping is required, like in cranes or hoists.

Dynamic braking employs the motor's own torque when it is disconnected from the supply of power. Instead of providing energy to the load, the motor's kinetic energy is converted into electrical energy that can be dissipated through resistive components, leading to a controlled reduction in speed.

Since all these mechanisms are used for braking motors in various applications, stating that "Any of these" is a valid braking method accurately captures the flexibility and diversity of techniques available to control motor speed.