What is an example of a negative feedback loop in humans?

A negative feedback loop is a crucial regulatory mechanism that helps maintain homeostasis within the body. In the context of the provided choices, the constriction of skin blood vessels due to cold exemplifies a negative feedback mechanism.

When the body is exposed to cold temperatures, it activates a response to conserve heat. Vasoconstriction, the narrowing of blood vessels, reduces blood flow to the skin and extremities, which helps retain heat within the core of the body. This reaction helps to maintain an optimal internal temperature by counteracting the effects of the cold environment, thereby illustrating how the body uses this mechanism to restore balance.

The other choices pertain to different physiological responses that are not classified as negative feedback. For instance, increased heart rate during exercise is a positive response to meet increased oxygen demands but does not counteract a disturbance. Likewise, an increased respiratory rate at high altitudes is an adaptation to lower oxygen levels rather than a feedback mechanism aiming to restore balance. Elevated insulin levels after eating reflect the body's response to increased blood glucose, assisting in lowering those levels, but this is part of a different regulatory process involving insulin rather than a classic negative feedback loop aimed at maintaining homeostasis.