ANS:A - final control element.

The mechanism which changes the value of the manipulated variable in response to the output signal from the control unit is called the final control element. Here's a brief explanation of each option:

1. Final control element: This is the component in a control system that directly adjusts the process in response to the control signal. It could be a valve, damper, motor, or any other device that alters the process conditions.
2. On-off control: On-off control is a type of control system where the final control element operates in either fully on or fully off states. It's a simple control mechanism suitable for systems where precise control isn't necessary.
3. Floating control action: Floating control action typically refers to a control system strategy where the final control element adjusts its position continuously in response to the process variable without fixed setpoints. It's often used in applications where maintaining a specific setpoint isn't critical.