**Core Concept**
The gate theory of pain is a physiological concept that explains how certain stimuli can modulate the transmission of pain signals to the brain. This theory proposes that certain nerve fibers can "gate" or block the transmission of pain signals to the brain, thereby reducing the perception of pain.

**Why the Correct Answer is Right**
The gate theory of pain was introduced by Ronald Melzack and Patrick Wall in 1965. According to this theory, certain nerve fibers, known as A-delta fibers, can inhibit the transmission of pain signals to the brain by stimulating the release of neurotransmitters such as substance P and GABA. This inhibition is thought to occur at the level of the spinal cord, where the A-delta fibers interact with other nerve fibers that transmit pain signals. Transcutaneous electrical nerve stimulation (TENS) is thought to work by stimulating the A-delta fibers, thereby activating the "gate" mechanism and reducing the transmission of pain signals to the brain.

**Why Each Wrong Option is Incorrect**
**Option A:** Allodynia refers to a condition in which a stimulus that is normally not painful is perceived as painful. While allodynia is a type of pain disorder, it is not directly related to the gate theory of pain or the mechanism of action of TENS.

**Option B:** Central pain refers to pain that arises from damage to the central nervous system, such as the brain or spinal cord. While central pain can be a result of various conditions, it is not directly related to the gate theory of pain or the mechanism of action of TENS.

**Option D:** Referred pain refers to pain that is perceived at a location other than the site of the painful stimulus. While referred pain can be a result of various conditions, it is not directly related to the gate theory of pain or the mechanism of action of TENS.

**Clinical Pearl / High-Yield Fact**
The gate theory of pain is an important concept in pain management, as it highlights the role of the nervous system in modulating the transmission of pain signals. TENS is a non-invasive and non-pharmacological technique that can be used to manage chronic pain by stimulating the A-delta fibers and activating the "gate" mechanism.

**✓ Correct Answer: C. Gate theory of pain**