Childbirth is an example of;

The answer is B) positive feedback.

A positive feedback mechanism is the complete opposite of a negative feedback mechanism. With negative feedback, the body's reaction reduces the original effect of the stimulus e.g. sweating to cool you down. In a positive feedback system, the reaction enhances the original stimulus e.g. during childbirth.
During labor, a hormone called oxytocin is released that intensifies and speeds up contractions. The increase in contractions causes more oxytocin to be released and the cycle goes on until the baby is born. The birth ends the release of oxytocin and ends the positive feedback mechanism. Another example is blood clotting.

Positive feedback occurs in two situations: childbirth (labor contractions), and clotting.

Positive feedback -- most feedback loops are negative (to shut off processes). This is one of the few examples of a positive one in the body.