**Core Concept**
The question is testing the knowledge of postmortem changes caused by specific types of poisoning. In forensic medicine, understanding these changes is crucial for determining the cause of death. The discoloration of the skin, known as livor mortis or postmortem discoloration, can provide valuable clues about the type of poisoning involved.

**Why the Correct Answer is Right**
Hydrogen sulfide poisoning is known to cause a greenish-blue discoloration of the skin due to the formation of sulfmethemoglobin. This occurs when hydrogen sulfide reacts with hemoglobin in red blood cells, leading to the formation of a stable compound called sulfmethemoglobin. This compound has a distinct greenish-blue color, which is a characteristic feature of hydrogen sulfide poisoning.

**Why Each Wrong Option is Incorrect**
**Option A:** Cyanide poisoning typically causes a pink or red coloration of the skin due to the inhibition of cytochrome c oxidase, which leads to an accumulation of oxygen in the tissues. This does not result in a greenish-blue discoloration.
**Option B:** Carbon monoxide poisoning causes a cherry-red coloration of the skin due to the formation of carboxyhemoglobin, which is a stable compound formed when carbon monoxide reacts with hemoglobin. This is in contrast to the greenish-blue discoloration seen in hydrogen sulfide poisoning.
**Option C:** Phosphorus poisoning can cause a variety of postmortem changes, including a greenish coloration of the skin, but this is not a specific or characteristic feature of phosphorus poisoning. The greenish coloration seen in phosphorus poisoning is more likely due to the formation of phosphorus-containing compounds in the tissues.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the color of the skin in a postmortem examination can provide valuable clues about the type of poisoning involved. However, it's also important to note that the color of the skin can be influenced by various factors, including the environment, the time elapsed since death, and the presence of other substances in the tissues. A thorough examination of the entire body, including the organs and tissues, is necessary to determine the cause of death accurately.

✓ Correct Answer: D. Hydrogen sulphide.