**Core Concept**
The greenish discoloration observed during putrefaction is a result of the breakdown of hemoglobin by bacteria, specifically by the enzyme sulfhemoglobin reductase. This process involves the reduction of sulfhemoglobin, a stable compound formed during the breakdown of hemoglobin, which contributes to the greenish color.

**Why the Correct Answer is Right**
The greenish discoloration is primarily due to the action of sulfhemoglobin reductase, an enzyme produced by bacteria that break down hemoglobin. This enzyme catalyzes the reduction of sulfhemoglobin, a compound that is resistant to further breakdown. As a result, sulfhemoglobin accumulates and contributes to the greenish color observed during putrefaction. The breakdown of hemoglobin by bacteria also leads to the production of other compounds, such as sulfides, which can contribute to the characteristic odor of putrefaction.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not specify the enzyme or process responsible for the greenish discoloration. While bacteria are involved in the breakdown of hemoglobin, the specific enzyme sulfhemoglobin reductase is not mentioned.
**Option B:** Incorrect because it does not accurately describe the process of putrefaction. While hemoglobin breakdown is involved, the greenish color is not due to the action of proteolytic enzymes.
**Option C:** Incorrect because it does not specify the enzyme or process responsible for the greenish discoloration. While bacteria are involved, the specific enzyme sulfhemoglobin reductase is not mentioned.

**Clinical Pearl / High-Yield Fact**
The greenish discoloration observed during putrefaction is a classic example of the breakdown of hemoglobin by bacteria, and the accumulation of sulfhemoglobin. This process is a key aspect of forensic pathology and can provide valuable information about the timing and circumstances of death.

**Correct Answer:** D.