Which system is described as having no feedback mechanism to adjust control actions?

An open-loop control system is characterized by its lack of a feedback mechanism to adjust control actions based on the output. In this type of system, the control inputs are set without considering the state of the output; the system simply executes the predetermined input commands to produce a specific output. This means that any disturbances or changes in the environment that may affect the output are not compensated for, leading to a system that operates solely on the initial parameters set at the beginning.

In contrast, a closed-loop control system actively monitors the output and makes adjustments based on the feedback received, which allows it to correct any errors and maintain desired performance. Hybrid and adaptive control systems integrate feedback and can adjust to changing conditions or optimize performance, further emphasizing the open-loop system's lack of a feedback mechanism. Thus, the defining feature of open-loop systems is their inability to self-correct or modify their actions based on the output, which makes this answer accurate.