**Core Concept**
Algor mortis, also known as the cooling of the body, is a post-mortem change that occurs after death. It is the rate at which the body temperature cools from the normal internal temperature of approximately 37°C (98.6°F) to the ambient temperature. Determining the time since death by algor mortis involves understanding the rate at which this cooling process occurs.

**Why the Correct Answer is Right**
The correct answer is based on the principle that the rate of algor mortis is influenced by the ambient temperature. The formula for determining the time since death by algor mortis is the temperature difference between the body and the ambient environment, divided by the rate of cooling per hour. This calculation assumes a constant ambient temperature and a cooling rate of 1.5°C (2.7°F) per hour in the first 12 hours after death. The formula is:

Time since death (hours) = (Body temperature - Ambient temperature) / Rate of cooling per hour

**Why Each Wrong Option is Incorrect**

**Option A:** This is incorrect because it does not take into account the rate of cooling per hour, which is a critical factor in determining the time since death by algor mortis.

**Option B:** This is incorrect because it suggests that the ambient temperature is not a factor in determining the time since death by algor mortis, which is not true.

**Option C:** This is incorrect because it implies that the time since death can be determined by a single measurement of body temperature, without considering the ambient temperature or the rate of cooling.

**Clinical Pearl / High-Yield Fact**
A common mnemonic to remember the order of post-mortem changes is "ALGOR": Algor (cooling of the body), Livor (discoloration of the skin), Putrefaction (decomposition of the body), and Rigor (stiffening of the body muscles). This mnemonic can help students remember the sequence of post-mortem changes that occur after death.

**Correct Answer:** D.