**Core Concept**
The question tests knowledge of toxicology, specifically the lethal dose of potassium cyanide, a potent inhibitor of cellular respiration. Potassium cyanide is a highly toxic substance that can cause death through inhibition of the **electron transport chain**. This understanding is crucial in medical toxicology.

**Why the Correct Answer is Right**
The correct answer is based on the fact that potassium cyanide is extremely toxic, with a lethal dose of approximately 1.4 to 2.5 milligrams per kilogram of body weight. This dose can vary depending on several factors, including the individual's weight and the method of ingestion. The **cyanide ion** binds to **cytochrome c oxidase**, preventing the utilization of oxygen in cellular respiration.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the dose is too low to be lethal for an average adult.
**Option B:** This option is also incorrect as it does not accurately reflect the known lethal dose range of potassium cyanide.
**Option D:** Similarly, this option is incorrect as the lethal dose of potassium cyanide is well established and not as high as this option suggests.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that cyanide poisoning requires immediate treatment, often involving **hydroxocobalamin** or other cyanide antidotes. Prompt recognition and intervention are key to preventing fatalities.

**Correct Answer:** C. 1.4 mg/kg