**Core Concept**
Electron microscopy requires methods to enhance contrast, allowing for the visualization of ultrastructural details. The specimen's inherent contrast is insufficient, necessitating the use of various staining techniques to achieve high-resolution images.

**Why the Correct Answer is Right**
Negative staining involves the application of a stain that does not penetrate the specimen but instead accumulates on its surface. This method improves the contrast of the specimen by creating a clear background and accentuating the specimen's morphology. The negatively charged stain molecules repel each other, resulting in a uniform distribution on the specimen's surface. This technique is particularly useful for visualizing the external morphology of cells and organelles.

**Why Each Wrong Option is Incorrect**
* **Option A:** Positive staining, which involves the penetration of the stain into the specimen, may not be as effective in improving contrast, as the stain may accumulate within the specimen rather than on its surface.
* **Option B:** En bloc staining, a technique used to stain the entire specimen before sectioning, may not provide the same level of contrast improvement as negative staining.
* **Option C:** The absence of staining altogether would result in a lack of contrast, making it difficult to visualize the specimen's ultrastructure.

**Clinical Pearl / High-Yield Fact**
Negative staining is a crucial technique in electron microscopy, allowing researchers to visualize the external morphology of cells and organelles with high contrast.

**Correct Answer: C. Negative staining improves the contrast of the specimen by creating a clear background and accentuating its morphology.**