Final answer:
A) Respiratory depression is the side effect associated with the activation of mu opioid receptors, which can be life-threatening due to the depressant effect on breathing control.
Step-by-step explanation:
The side effects associated with the activation of mu opioid receptors include a variety of physiological changes due to the action of opioids on the central nervous system. One of the most significant and potentially dangerous side effects of opioid receptor activation is respiratory depression. This occurs because opioids have a depressant effect on the part of the brain that controls breathing, which can lead to slower and more shallow respiration. It's important to note that while other symptoms such as euphoria or relaxation can occur, hallucinations and increased heart rate are not common opioid side effects, nor is reduced appetite specifically linked to mu receptor activation. Naloxone, an opiate antagonist, can quickly reverse respiratory depression by displacing opioids from their receptors, underscoring the role of mu receptors in this adverse effect.