Final answer:
A device that prevents the application of a counter torque until the motor speed is reduced to an acceptable value is known as a Brake, which is essential for safely slowing down or stopping mechanical systems. The correct option is B.
Step-by-step explanation:
When a device prevents the application of a counter torque until the motor speed is reduced to an acceptable value, it is known as a Brake. The function of a brake is to slow down or stop the motion of a mechanical system. This principle can be observed in various scenarios.
For example, a hybrid car charges its batteries as it decelerates through regenerative braking, where the kinetic energy of the vehicle is converted back into electrical energy to charge the battery.
Similarly, airplane flaps use air resistance to slow down the aircraft during landing. These mechanisms are all related to the concept of controlling the speed of a system by applying a counterforce. Option B. is the correct one.