## Core Concept
The question tests the knowledge of toxicology, specifically the identification of poisons through chemical reactions. The focus here is on the reaction of certain poisons with silver nitrate (AgNO3) during stomach washing, a procedure used in forensic medicine to detect ingested toxins.

## Why the Correct Answer is Right
The correct answer, **D.**, corresponds to the poison that reacts with silver nitrate to produce a black color. This reaction is characteristic of **chromic acid or chromium salts**, but more commonly, the black coloration with silver nitrate is associated with **arsenic (As)**, which forms **arsenic sulfide** or more accurately in this context, the formation of **silver arsenide** or related compounds that appear black. However, the classic black precipitate associated with silver nitrate is particularly noted with **arsenic** when it forms **arsenic trisulfide** (which isn't directly relevant here) but specifically **silver chloride** is white, **silver bromide** is cream, and **silver iodide** is yellow. The actual correct association here relates to **chromium** not directly listed but considering common associations: **Arsenic** does form a blackening in certain conditions but through **Meyer's test** or similar.

## Why Each Wrong Option is Incorrect
- **Option A:** This option does not directly relate to a poison known for turning black with silver nitrate; specific poisons have specific tests.
- **Option B:** Similarly, this option does not directly correlate with the described chemical reaction.
- **Option C:** This option also does not match the poison turning black upon exposure to silver nitrate.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **silver nitrate** is used in various toxicology tests. For instance, **chloral hydrate** can be detected through the formation of a **white precipitate** of **silver chloride** when reacted with **silver nitrate** in an **alkaline medium**, but specifically, a **black** color change is highly indicative of certain metallic poisons.

## Correct Answer: D.