**Core Concept**
The greenish discoloration observed during putrefaction is a result of the breakdown of hemoglobin and the presence of certain bacteria. This process involves the decomposition of tissues and the release of gases, including hydrogen sulfide and methane.

**Why the Correct Answer is Right**
The greenish discoloration is primarily caused by the production of sulfhemoglobin, a compound formed when hydrogen sulfide reacts with hemoglobin. This reaction is facilitated by the enzyme sulfite oxidase and certain bacteria such as Clostridium perfringens, which are commonly found in the gut. The presence of sulfhemoglobin is responsible for the greenish color observed during putrefaction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specifically relate to the greenish discoloration observed during putrefaction.
**Option B:** This option is incorrect because it refers to the brownish discoloration observed in cases of carbon monoxide poisoning, not putrefaction.
**Option C:** This option is incorrect because it refers to the yellowish discoloration observed in cases of jaundice, not putrefaction.

**Clinical Pearl / High-Yield Fact**
In forensic pathology, the color of the skin during putrefaction can provide valuable clues about the post-mortem interval and the conditions under which the body was left.

**Correct Answer:** D.