**Core Concept**
Postmortem staining, also known as livor mortis, is the discoloration of the skin and underlying tissues due to the pooling of blood in the venous system after death. This phenomenon is influenced by gravity and the breakdown of cellular integrity.

**Why the Correct Answer is Right**
Postmortem staining typically becomes fixed within 8-12 hours after death, as the blood in the venous system coagulates and becomes trapped, leading to a permanent discoloration of the affected area. The exact timing can vary depending on environmental factors such as temperature, but 8-12 hours is generally considered the window for postmortem staining to become fixed.

**Why Each Wrong Option is Incorrect**
**Option A:** 2 hours - This is too early for postmortem staining to become fixed, as the blood has not had sufficient time to coagulate and become trapped in the venous system.
**Option B:** 24 hours - While postmortem staining can persist for an extended period, it does not become fixed within 24 hours, as the body's decomposition process can cause further changes in tissue coloration.
**Option C:** 6 hours - This is within the range where postmortem staining is still in the process of developing, rather than becoming fixed.

**Clinical Pearl / High-Yield Fact**
It is essential to note that postmortem staining can be influenced by various factors, including environmental conditions, the presence of underlying medical conditions, and the manner of death. A thorough postmortem examination is necessary to determine the cause and timing of death.

**Correct Answer: D. 12 hours**