**Core Concept**
The question is asking about the preservative used for vitreous in autopsy, which is a key concept in forensic pathology. The vitreous humor is a clear gel-like substance in the eye, and its analysis is crucial in determining the cause of death, particularly in cases of suspected poisoning or trauma.

**Why the Correct Answer is Right**
In autopsy, the vitreous humor is often analyzed for its potassium and chloride levels, which can provide valuable information about the deceased's state of hydration and potential toxic exposures. The preservative used to collect and store the vitreous humor should minimize any changes in its composition, allowing for accurate analysis. The correct preservative used for vitreous in autopsy is formaldehyde, which is a common fixative used in histopathology and forensic pathology.

**Why Each Wrong Option is Incorrect**
**Option A:** Formalin is a mixture of formaldehyde and other substances, but it is not the specific preservative used for vitreous in autopsy.
**Option B:** Glutaraldehyde is a fixative used in histopathology, but it is not typically used for vitreous humor.
**Option C:** Saline solution is often used as a preservative for other tissues, but it is not suitable for vitreous humor due to its high water content.

**Clinical Pearl / High-Yield Fact**
Remember that the analysis of vitreous humor is a critical component of forensic pathology, and the use of the correct preservative is essential to ensure accurate results.

**Correct Answer: D. Formaldehyde**