## Core Concept
The question pertains to the preservative used in cases of alcohol poisoning, specifically in the context of forensic or clinical toxicology. Preservatives are added to samples to prevent degradation of the alcohol or other substances, ensuring accurate analysis.

## Why the Correct Answer is Right
The correct answer, **D. Potassium dichromate**, is a well-known preservative used in alcohol poisoning cases. Potassium dichromate acts by preventing the microbial fermentation of alcohol in the sample, thereby preserving the alcohol content for accurate measurement. This is crucial in forensic analysis where the exact concentration of alcohol can have legal implications.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is blank and does not provide a viable choice for consideration.
- **Option B:** This option is also blank and does not offer a valid alternative.
- **Option C:** Similarly, this option is blank and lacks a specific substance for evaluation.

## Clinical Pearl / High-Yield Fact
A key point to remember is that in cases of suspected alcohol poisoning or when measuring blood alcohol content for forensic purposes, preserving the sample is critical. **Potassium dichromate** is specifically used for this purpose because it inhibits microbial growth and prevents the breakdown of alcohol in the sample.

## Correct Answer Line
**Correct Answer: D. Potassium dichromate.**