**Core Concept**
A Pap Smear is a cytological test used for the early detection of cervical cancer and its precursors. The test involves collecting cells from the cervix and examining them under a microscope for abnormal cell changes. To prepare the cells for examination, they are fixed onto a slide using a chemical process.

**Why the Correct Answer is Right**
The Pap Smear cells are fixed onto a slide using a solution of 95% ethanol, which acts as a fixative to preserve the cellular morphology. The ethanol denatures the proteins in the cells, preventing them from breaking down and losing their shape. This allows the cells to be stained and examined under a microscope for abnormal cell changes. The fixation process is critical in preserving the cellular details, enabling accurate diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Formaldehyde is not typically used as a fixative for Pap Smears, as it can cause the cells to become over-fixed and difficult to stain.
**Option B:** Acetone is not suitable for fixing Pap Smear cells, as it can cause the cells to become brittle and prone to cracking.
**Option C:** Water is not an effective fixative for Pap Smear cells, as it allows the cells to break down and lose their morphology.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the Pap Smear test is not a diagnostic tool, but rather a screening test. Any abnormal cell changes detected on a Pap Smear require further investigation, such as a colposcopy or biopsy, to confirm the diagnosis.

**Correct Answer:** C. 95% ethanol.