**Core Concept**
The fixative used in histopathology is a chemical substance that helps preserve tissue samples by preventing decay, autolysis, and distortion, making it possible to examine the tissue under a microscope. Fixatives work by cross-linking proteins and stabilizing cellular structures, thereby preserving the tissue's morphology.

**Why the Correct Answer is Right**
The most commonly used fixative in histopathology is formalin, a solution of formaldehyde in water. Formalin works by cross-linking proteins through a process called aldehyde fixation, which involves the formation of covalent bonds between amino groups of proteins. This fixation process helps preserve the tissue's cellular and architectural details, making it suitable for histological examination. Formalin fixation is widely used in pathology labs due to its effectiveness and ease of use.

**Why Each Wrong Option is Incorrect**
* **Option A:** Glutaraldehyde is a fixative used in electron microscopy, not in routine histopathology.
* **Option B:** Ethanol is a dehydration agent used to preserve tissue samples before embedding, but it is not a fixative.
* **Option C:** Propylene oxide is a solvent used in electron microscopy, not a fixative.

**Clinical Pearl / High-Yield Fact**
Formalin fixation is a crucial step in histopathology, but it can also lead to artifacts, such as shrinkage and distortion of tissue samples. It's essential to follow proper fixation protocols to ensure accurate histological examination.

**Correct Answer: C. Propylene oxide is not a fixative used in histopathology.