The correct answer is D) endogenous.
Chemicals produced within the brain as a reaction to severe pain or intense exercise are intrinsically produced. Endogenous substances are naturally produced within the body, in this case, the brain, in response to specific stimuli or conditions. In the context of severe pain or intense exercise, the brain releases endogenous chemicals called endorphins.
Endorphins are neurotransmitters that relieve pain and enhance mood. They are part of the body's natural pain-relief system and help alleviate pain and induce pleasure or euphoria. Endorphins play a crucial role in regulating pain perception and are often associated with the "runner's high" experienced during intense exercise.
Exogenous substances, on the other hand, are substances that are introduced into the body from outside sources. Placebos, as mentioned in option B, are inert substances that have no therapeutic effect but may produce a positive response due to psychological factors. Therefore, the correct answer is D) endogenous when considering the chemicals produced within the brain in response to severe pain or intense exercise.