**Core Concept**
The preservation of protozoal cysts requires a method that maintains their morphology and viability for diagnostic purposes. This is crucial in identifying and differentiating various species of protozoa in clinical and laboratory settings.

**Why the Correct Answer is Right**
The best preservative for protozoal cysts is formaldehyde, which is a common fixative used in microscopy. Formaldehyde works by cross-linking proteins and preserving the cellular structure, thereby maintaining the integrity of the cysts for examination. This is essential in diagnosing diseases caused by protozoa, such as Giardia and Cryptosporidium. Formaldehyde is effective in preserving the morphology of protozoal cysts, allowing for accurate identification and diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** None of the other options are correct, as formaldehyde is the preferred preservative for protozoal cysts.
**Option B:** Mercuric chloride is not suitable for preserving protozoal cysts, as it can cause damage to the cellular structure and is toxic.
**Option C:** Glycol is not effective in preserving protozoal cysts, as it can cause degradation of the cellular components and is not a suitable fixative.
**Option D:** Normal saline is not a suitable preservative for protozoal cysts, as it can cause lysis of the cells and is not effective in maintaining their morphology.

**Clinical Pearl / High-Yield Fact**
When preserving protozoal cysts, it is essential to use a fixative that maintains their morphology and viability. Formaldehyde is the preferred preservative due to its ability to cross-link proteins and preserve the cellular structure.

**Correct Answer:** D. Formaldehyde is not correct in the given options.