The correct option that relates to the principle of a feedback loop and control of one of the variables is:
C) Closed Loop System; Pressure Control
In a closed-loop system, feedback is utilized to adjust the system's output based on the difference between the desired setpoint and the actual output. In the context of pressure control, sensors measure the current pressure, compare it to the desired pressure (setpoint), and provide feedback to control mechanisms that adjust inputs (like valves or pumps) to maintain the pressure within the desired range. This closed-loop process helps maintain stability and precision in controlling the pressure variable.