The correct answer is C) Feedback. In a closed-loop process control system, the term used to describe the action of measuring the difference between the actual result and the desired result, and using that difference to drive the actual result toward the desired result, is feedback. Feedback is a critical component of closed-loop control systems, as it allows the system to constantly monitor and adjust its output to maintain the desired performance. The other answer choices are incorrect because: A) Instability refers to the tendency of a system to oscillate or become unstable, which is the opposite of the desired behavior in a closed-loop control system. B) Gain refers to the amplification factor of a system, which is a separate concept from the feedback mechanism. D) Dead band refers to a range of input values where the system does not respond, which is also not the same as the feedback mechanism.