Final answer:
The correct answer to the question is that opioid use in the 12-week primary outcomes of the studies showed an improvement in physical functioning (C), although overall health remained worse than that of the general population.
Step-by-step explanation:
The correct answer about opioid use in the 12-week primary outcomes of the studies is C) Improved physical functioning. Monthly survey data have shown improvements in physical health for those in treatment, although it is important to note that clients' mental and physical health were still significantly worse than that of the general public. This improvement is critical in the context of the opioid crisis, where effective treatment can lead to better health outcomes. Opioid withdrawal symptoms can include severe pain, which may be why a person with a heroin addiction might be prescribed methadone, a psychoactive drug that helps to reduce withdrawal symptoms and cravings, potentially preventing relapse.