**Core Concept:**
Nervous tissue is a type of tissue in the body that comprises neurons, glial cells, and other supporting cells, and plays a vital role in transmitting signals within the body. Damage to nervous tissue can occur due to various reasons like trauma, infections, autoimmune disorders, etc. These damages may lead to functional loss or disruption of communication between neurons, which must be repaired to restore the normal physiological function.

**Why the Correct Answer is Right:**
The correct answer, **D.** Neural stem cells, plays a crucial role in repairing damage to nervous tissue. Neural stem cells are multipotent cells that have the ability to differentiate into neurons and glial cells under certain conditions. When damaged tissue is provided with the right signals, these stem cells migrate to the site of injury, proliferate, and differentiate into the required cell types to replace the damaged cells.

**Why Each Wrong Option is Incorrect:**

A. **Neurons:** Neurons are terminally differentiated cells and cannot regenerate, unlike stem cells. Therefore, neurons cannot repair damaged nervous tissue.

B. **Glial cells:** Glial cells are non-neuronal cells that provide structural and supportive roles to neurons, but they cannot regenerate like stem cells.

C. **External transplantation:** This option suggests transplanting healthy neurons or glial cells from an external source to replace the damaged ones. Although this approach has shown promise in animal models, it is not yet a standard method for treating nervous tissue damage in humans.

**Clinical Pearl:**
Understanding the role of neural stem cells in nervous tissue repair is crucial for future therapeutic strategies aimed at repairing damaged nervous tissue in humans. While current approaches are limited, advances in stem cell biology and tissue engineering may lead to innovative treatments for neurological disorders in the future.