What action would change on a receiver in a control system?

In a control system, the term "gain" refers to the ratio of the output signal to the input signal within the system, which determines how responsive the system is to changes in the input. When gain is adjusted on a receiver, it effectively changes how much the controller reacts to variations in the input signal.

For instance, increasing the gain will cause the system to respond more vigorously to input changes, thereby amplifying the output. Conversely, reducing the gain would make the system's response to input changes less sensitive. This is particularly crucial in process control where maintaining specific settings or conditions is vital.

The other options relate to different facets of the control system. Setting the "set point" determines what the desired target value is for the variable being controlled. "Range" refers to the span of values that the system can handle, defining its limits. The "mode" indicates the operational state of the system (such as manual or automatic). While all these factors can influence the performance and functioning of a control system, it is the gain that directly affects the responsiveness of the receiver to input changes, making it the correct choice.