**Core Concept**
Rigor mortis is the stiffening of muscles that occurs after death, caused by a series of complex biochemical reactions involving energy depletion, ionic shifts, and protein denaturation. This process is essential for forensic pathology and is used in the estimation of post-mortem interval.

**Why the Correct Answer is Right**
The correct answer is associated with the onset of rigor mortis, which begins with the depletion of ATP (adenosine triphosphate) in muscle cells. As ATP levels drop, actin and myosin filaments in muscle cells undergo a series of conformational changes, leading to the formation of rigid cross-links between actin and myosin. This cross-linking results in the characteristic stiffness of rigor mortis. The process typically begins within 2-4 hours after death.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect as the process begins after the initial energy depletion; thus, it is not the first stage of rigor mortis.
**Option B:** This is incorrect as it is not associated with the onset of rigor mortis; rather, it is a consequence of the process.
**Option C:** This is incorrect as it is not directly related to the biochemical changes that lead to rigor mortis.

**Clinical Pearl / High-Yield Fact**
Rigor mortis typically reaches its peak within 12-24 hours after death and begins to resolve as the muscle cells begin to autolyze, breaking down the cross-links between actin and myosin.

**Correct Answer: D.**