**Core Concept**
Firearm injury entry-wound blackening is a phenomenon where the entrance wound exhibits charring, tattooing, or blackening due to the deposition of unburnt gunpowder particles or soot. This occurs as a result of the high-temperature combustion products generated during the propulsion of the projectile.

**Why the Correct Answer is Right**
The correct answer is due to the fact that gunpowder is a high-nitrogen, carbon-based explosive that produces a large amount of soot and particulate matter when ignited. When a firearm is discharged, the combustion products, including unburnt gunpowder particles, are propelled out of the barrel along with the projectile. If these particles come into contact with the skin, they can deposit and cause charring or tattooing, leading to the characteristic blackening of the entry wound.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because the primary cause of entry-wound blackening is not the heat generated by the firearm, but rather the deposition of unburnt gunpowder particles.
* **Option B:** Incorrect because the type of firearm used (e.g., handgun, rifle) does not affect the presence or extent of entry-wound blackening.
* **Option C:** Incorrect because the size of the projectile does not influence the amount of gunpowder deposited on the skin.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the presence and extent of entry-wound blackening can provide valuable information about the distance of the firearm from the victim's body at the time of discharge, with closer proximity leading to more pronounced blackening.

**Correct Answer: . Gunpowder deposition.**