**Core Concept**
Transcutaneous electrical nerve stimulation (TENS) is a non-invasive pain relief technique that uses low-voltage electrical currents to stimulate nerves and block pain signals to the brain. It works by activating large nerve fibers, which then inhibit the transmission of pain signals from small nerve fibers.

**Why the Correct Answer is Right**
TENS works by stimulating the A-beta fibers, which are large nerve fibers responsible for transmitting touch and pressure sensations. When these fibers are stimulated, they release neurotransmitters that activate inhibitory interneurons in the spinal cord, which in turn inhibit the transmission of pain signals from small nerve fibers, such as A-delta and C-fibers. This results in a reduction in pain perception.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because TENS does not primarily work by stimulating the A-delta fibers, which are responsible for transmitting sharp, stabbing pain sensations.
**Option B:** This option is incorrect because TENS does not primarily work by stimulating the C-fibers, which are responsible for transmitting slow, burning pain sensations.
**Option C:** This option is incorrect because TENS does not primarily work by stimulating the sympathetic nervous system, which is responsible for the "fight or flight" response.

**Clinical Pearl / High-Yield Fact**
It's essential to note that TENS is not suitable for all types of pain, particularly neuropathic pain, and should be used under the guidance of a healthcare professional. Additionally, the effectiveness of TENS can vary depending on the individual and the specific condition being treated.

**Correct Answer:** D.