**Core Concept**
Electron microscopy requires a rapid fixation method to preserve the ultrastructure of cells and tissues. The fixative used must be able to penetrate the tissue quickly, cross-link proteins, and prevent autolysis and enzymatic degradation.

**Why the Correct Answer is Right**
Glutaraldehyde is the most common fixative used in electron microscopy due to its high penetration rate and ability to cross-link proteins, resulting in excellent preservation of ultrastructural details. It works by reacting with amino groups on proteins, forming covalent bonds and stabilizing the tissue structure. Glutaraldehyde is particularly effective for preserving the fine details of cell membranes, cytoskeletal structures, and organelles.

**Why Each Wrong Option is Incorrect**
**Option A:** Formaldehyde is used in light microscopy but is not suitable for electron microscopy due to its low penetration rate and inability to preserve ultrastructural details.
**Option B:** Paraformaldehyde is a polymer of formaldehyde and is used in some electron microscopy protocols, but it is not as effective as glutaraldehyde for preserving ultrastructural details.
**Option C:** Acrolein is a fixative used in some electron microscopy protocols, but it is not as widely used as glutaraldehyde and can be toxic to some tissues.

**Clinical Pearl / High-Yield Fact**
Glutaraldehyde is a potent fixative that must be handled with care due to its toxicity and ability to cause skin and respiratory irritation.

**Correct Answer:** C. Glutaraldehyde.