**Core Concept**
Post mortem staining, also known as livor mortis, is the discoloration of the skin due to the pooling of blood in the dependent areas following death. This phenomenon occurs due to the gravitational settling of blood in the microcirculation, which is facilitated by the breakdown of cellular and vascular integrity upon death.

**Why the Correct Answer is Right**
Livor mortis becomes fixed after 6-7 hours due to the coagulation of blood within the microcirculation. This process involves the conversion of fibrinogen to fibrin, which forms a gel-like substance that stabilizes the blood in its new position. As a result, the coloration of the skin becomes fixed and cannot be easily altered by changes in body position. This fixed staining can be used as an important indicator of the post mortem interval in forensic investigations.

**Why Each Wrong Option is Incorrect**
**Option A:** 2-3 hours is too early for livor mortis to become fixed, as the blood has not had sufficient time to coagulate in the microcirculation.
**Option B:** 3-4 hours is still within the range where livor mortis can be easily altered by changes in body position, making it an unreliable indicator of the post mortem interval.
**Option C:** 5-6 hours is closer to the correct answer, but the blood has not yet fully coagulated, and the staining may still be subject to some degree of change.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the coloration of livor mortis can vary depending on the individual's skin pigmentation and the presence of underlying medical conditions. However, in general, the fixed nature of the staining after 6-7 hours provides a reliable indicator of the post mortem interval.

**✓ Correct Answer: D. 6-7 hrs.**