Human eye has a biological 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

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.

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the answer would be d : p