## Core Concept
The question pertains to the pharmacology of cyanide, specifically asking about a non-toxic salt of cyanide. Cyanide is a potent toxin that inhibits cellular respiration by binding to cytochrome c oxidase in mitochondria. Various salts of cyanide have different levels of toxicity.

## Why the Correct Answer is Right
The correct answer, **. (Potassium ferrocyanide)**, is a non-toxic salt of cyanide because the cyanide ions are tightly bound within the ferrocyanide complex. This complex is stable and does not readily release free cyanide ions, which are responsible for the toxic effects of cyanide. Therefore, despite containing cyanide, potassium ferrocyanide is not toxic.

## Why Each Wrong Option is Incorrect
- **Option A:** Sodium cyanide (NaCN) is highly toxic as it readily dissociates into sodium and cyanide ions, releasing free cyanide that can inhibit cellular respiration.
- **Option B:** Hydrogen cyanide (HCN) is also highly toxic, existing as a gas or liquid, and can easily release cyanide ions or gas, which is quickly absorbed through the skin, lungs, or gastrointestinal tract.
- **Option D:** Calcium cyanide (Ca(CN)2) is toxic for the same reasons as sodium cyanide; it dissociates into calcium and cyanide ions, releasing free cyanide.

## Clinical Pearl / High-Yield Fact
A key point to remember is that while certain compounds like **potassium ferrocyanide** have cyanide bound in a way that is not toxic, the treatment of cyanide poisoning often involves converting hemoglobin to methemoglobin, which can bind cyanide, or using substances like hydroxocobalamin that can directly bind cyanide.

## Correct Answer: . Potassium ferrocyanide