What refers to the instruments that take action to adjust process variables?

The term that refers to instruments that take action to adjust process variables is a control device. These devices are integral to automated systems and play a crucial role in maintaining desired process conditions by adjusting inputs based on the measurements they receive.

Control devices continuously monitor the process variables, such as temperature, pressure, or flow rate. When they detect deviations from set points or desired values, they execute commands to correct these deviations, which can involve opening or closing valves, adjusting pumps, modulating airflow, and other necessary actions. This capability is essential for ensuring that processes operate within safe and efficient ranges.

In contrast, transmitters serve to convert physical parameters into signals that can be understood by controllers or monitoring systems. Local and remote indicators display process variable measurements but do not have the capability to adjust those variables. Thus, while all the other options refer to important components in an instrumentation system, only a control device actively modifies the process conditions based on the information it receives.