**Core Concept**
Rigor mortis is the temporary stiffening of muscles that occurs after death due to a series of complex biochemical reactions involving energy failure, ATP depletion, and actin-myosin filament interactions.

**Why the Correct Answer is Right**
After death, the energy-dependent process of muscle relaxation is disrupted, leading to a rapid increase in intramuscular calcium levels. This triggers a series of enzymatic reactions that ultimately result in the precipitation of actin and myosin filaments, causing muscle stiffness. The earliest affected muscles are typically those with the highest ATP turnover rates, such as those in the face and neck. The process of rigor mortis begins within 2-4 hours post-mortem and reaches its peak around 12-24 hours.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because rigor mortis does not occur due to a decrease in ATP levels, but rather due to an increase in intramuscular calcium and subsequent actin-myosin filament interactions.
**Option B:** This option is incorrect because rigor mortis is not a result of lactic acid buildup, but rather a consequence of ATP depletion and subsequent biochemical reactions.
**Option C:** This option is incorrect because the onset of rigor mortis is not influenced by the ambient temperature, although it may be affected by the rate of post-mortem decay.

**Clinical Pearl / High-Yield Fact**
It is essential to note that rigor mortis can be delayed or prevented in certain circumstances, such as in cases of hypothermia, drowning, or in individuals with certain neuromuscular disorders.

**Correct Answer: C. 2-4 hours post-mortem**