Answer;
-Natural control system or biological system
Explanation;
-Natural control system is a classification of control systems. The systems inside a human being or a biological system are known as natural control systems.
-For instance, the human eye has a biological (or natural) control system that varies the pupil  diameter to maintain constant light intensity to the retina. As the light intensity increases, the  optical nerve sends a signal to the brain, which commands internal eye muscles to decrease the  pupil’s eye diameter. When the light intensity decreases, the pupil diameter increases.
-Other control systems include; man-made control system, combinational control system and also time-varying and time-invariant control systems.
