Which of the following describes an explosion?

An explosion is characterized by the sudden violent expansion of gases. This rapid expansion occurs when a significant amount of energy is released, typically due to a chemical reaction or physical change, causing a dramatic increase in pressure and temperature. In this context, the explosion leads to an expansive force that can create shock waves and significant damage, which distinguishes it from other processes involving combustion or pressure.

In contrast, the other options describe processes that do not involve the rapid and violent release of energy typical of an explosion. A slow combustion process refers to a controlled burn that takes place over time and does not yield the sudden release of gases or energy. A gradual build-up of gas pressure can potentially lead to an explosion if it exceeds containment limits, but by itself, it does not define an explosion; it merely sets the stage for one. Finally, a chemical reaction that produces heat without gas release describes exothermic reactions that do not produce an expansive effect typical of explosive events. Each of these illustrates distinct processes that do not encapsulate the definition of an explosion as effectively as the sudden, violent expansion of gases does.