A. Endorphins block pain signals by mimicking opioid drugs. TENS therapy stimulates the release of endorphins, which are endogenous opioids. They bind to opioid receptors and inhibit pain signal transmission-similar to how opioid medications work. B. Endorphins enhance the release of serotonin, leading to pain reduction. Although serotonin plays a role in pain modulation, TENS primarily works through endorphin and enkephalin release, not via serotonin enhancement. C. Endorphins increase the production of norepinephrine to reduce pain. Norepinephrine also plays a role in descending pain inhibition, but this is not the main mechanism triggered by TENS. D. Endorphins stimulate nerve cells to increase pain sensation. This is the opposite of their function-endorphins reduce pain perception, not increase it.