## Core Concept
Rigor mortis is a well-known post-mortem change that occurs in the muscles of a deceased individual, leading to temporary stiffening of the body. It has significant medicolegal importance, particularly in estimating the **post-mortem interval (PMI)**, which is the time elapsed since death.

## Why the Correct Answer is Right
The correct answer, , is related to the estimation of the post-mortem interval. Rigor mortis typically sets in around 2-4 hours after death, peaks at 12-18 hours, and resolves by 24-36 hours. By analyzing the progression and resolution of rigor mortis, forensic experts can make an educated estimate of the time since death occurred. This information is crucial in criminal investigations and medicolegal proceedings.

## Why Each Wrong Option is Incorrect
* **Option A:** - This option is incorrect because while algor mortis (cooling of the body) and livor mortis (discoloration due to blood settling) are also post-mortem changes, they are not directly related to rigor mortis.
* **Option B:** - This option is incorrect as supravital reaction refers to changes occurring in the muscles or other tissues shortly after death but before the onset of rigor mortis. Though important, it's not the primary focus of rigor mortis in medicolegal contexts.
* **Option C:** - This option is incorrect because the cause of death is determined through a combination of autopsy findings, toxicology, and medical history, not through the analysis of rigor mortis.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the timing of rigor mortis can be influenced by environmental factors such as temperature. Higher temperatures can accelerate the onset and resolution of rigor mortis, while lower temperatures can delay it. This variability underscores the need for careful observation and correlation with other post-mortem changes when estimating the post-mortem interval.

## Correct Answer: D. Post-mortem interval.