The correct answer is - D. Endorphins.
The endorphins are the natural way of our bodies to relieve from stress and pains. In essence, the endorphins are a hormone released mostly by the brain, but also by the other parts of the nervous system.
In a way, we can say that the endorphins are the natural opiate of the body, helping us to feel good, or at least better, in situations where the body is under certain stress or pain.