A firearm is a mechanical device that uses pressure from _________ to force a projectile through and out of a metal tube.

The correct answer focuses on the fundamental operation of firearms, which utilize chemical energy generated by burning powder to propel a projectile. When the powder in the cartridge ignites, it produces a rapid expansion of gas as it burns. This explosion creates high pressure within the chamber of the firearm, allowing the projectile to be forcibly expelled through the barrel.

This choice emphasizes the primary component involved in nearly all modern ammunition types, where the burning powder is essential in generating the explosive force needed for firing.

Other options, while they may suggest methods of propulsion or pressure, do not accurately reflect the mechanism used in typical firearms. Compressed air is generally associated with air guns and does not align with conventional firearms that rely on combustion. The mention of a steel rod does not pertain to the operation of firearms, as it is not a factor in the propulsion system. The choice of an explosion, while closely related, is more generalized and does not specify the use of powder, which is critical for understanding the mechanics of firearms.