**Core Concept**
The estimation of time since death is a crucial aspect of forensic pathology, and various methods have been developed to estimate the post-mortem interval (PMI). The ideal medium for estimating the PMI should be easily accessible, non-invasive, and provide a clear indication of the elapsed time.

**Why the Correct Answer is Right**
The correct answer is **C. Stomach contents**. The stomach contents, particularly the digestion of food, can be used to estimate the time since death. The digestion process is influenced by the type of food consumed, the pH of the stomach, and the temperature. The digestion of food in the stomach follows a predictable pattern, and by analyzing the stomach contents, forensic pathologists can estimate the time elapsed since death. This method is particularly useful in cases where other methods, such as body temperature or rigor mortis, are unreliable or impossible to use.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood alcohol levels can be used to estimate the time since death, but they are not as reliable as stomach contents, as they can be influenced by various factors, such as the amount of alcohol consumed and the individual's metabolism.

**Option B:** Body temperature can be used to estimate the time since death, but it is not as reliable as stomach contents, as it can be influenced by various factors, such as the ambient temperature and the individual's physical condition.

**Option C:** Rigor mortis can be used to estimate the time since death, but it is not as reliable as stomach contents, as it can be influenced by various factors, such as the individual's physical condition and the ambient temperature.

**Option D:** The presence of flies can be used to estimate the time since death, but it is not as reliable as stomach contents, as it can be influenced by various factors, such as the ambient temperature and the presence of other animals.

**Clinical Pearl / High-Yield Fact**
The digestion of food in the stomach follows a predictable pattern, and by analyzing the stomach contents, forensic pathologists can estimate the time elapsed since death. This method is particularly useful in cases where other methods, such as body temperature or rigor mortis, are unreliable or impossible to use.

**Correct Answer: C. Stomach contents**