**Core Concept**
The onset and duration of algor mortis (the cooling of the body after death) are influenced by several factors, but the major factor is the ambient temperature. This is because the body temperature gradually equilibrates with the surrounding environment, leading to a decrease in body temperature.

**Why the Correct Answer is Right**
The ambient temperature plays a crucial role in determining the rate of heat loss from the body. As the body temperature cools, the metabolic rate and blood circulation decrease, leading to a decrease in heat production. The rate of heat loss is determined by the temperature gradient between the body and the environment, with higher ambient temperatures resulting in slower cooling rates. This is why algor mortis is often used as an indicator of the post-mortem interval, with the body temperature being used to estimate the time elapsed since death.

**Why Each Wrong Option is Incorrect**
* **Option A:** is incorrect because while the body's clothing can influence the rate of heat loss, it is not the major factor influencing the onset and duration of algor mortis.
* **Option B:** is incorrect because the body's internal temperature regulation mechanisms, such as sweating and shivering, are no longer functioning after death and therefore do not influence the onset and duration of algor mortis.
* **Option C:** is incorrect because the body's composition, such as the amount of fat or muscle mass, can influence the rate of heat loss, but it is not the major factor influencing the onset and duration of algor mortis.

**Clinical Pearl / High-Yield Fact**
It is essential to note that algor mortis is not a reliable indicator of the post-mortem interval in cases where the body has been moved to a different environment, as the ambient temperature can significantly influence the rate of cooling.

**Correct Answer: A. Ambient temperature**