Which is a feedback mechanism for a system?
the center scroll wheel on a computer mouse
the temperature setting knob on a refrigerator
O the power button on a computer monitor
the process bar when downloading a file on iTunes

The following is a feedback mechanism for a system :

The progress bar when downloading a file on iTunes

Explanation:

A feedback mechanism is a loop system wherein the system responds to a perturbation. The response may be in the same direction (as in positive feedback) or in the opposite direction (as in negative feedback).Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to feed back into itself.Evaluation feedback needs to be done “in the moment” to help the person receiving the feedback know where they stand.A feedback control system consists of five basic components: (1) input, (2) process being controlled, (3) output, (4) sensing elements, and (5) controller and actuating devices.Because negative feedback produces stable circuit responses, improves stability and increases the operating bandwidth of a given system, the majority of all control and feedback systems is degenerative reducing the effects of the gain.