When a variation outside normal limits triggers a response that restores the normal condition, the regulatory mechanism

(a) negative feedback.
(b) positive feedback.
(c) compensation.
(d) adaptation.

(a) negative feedback.

Explanation:

A negative feedback mechanism operates in the direction of maintaining the homeostasis. Any change from the defined range of internal conditions of the body is sensed as a stimulus. It triggers a response that restores the original range of the internal condition. This is called a negative feedback mechanism. For example, when the body temperature rises, the blood vessels present near the skin surface dilate to lose more heat to the environment. This is one of the negative feedback produced against raised body temperature.