What creates the force when an explosion is fired?

The correct choice highlights that the force generated during an explosion is primarily the result of the sudden expansion of gases that are released very quickly. In an explosion, a chemical reaction occurs that produces gas at a rapid rate, leading to a significant increase in volume and pressure. This sudden expansion results in a shockwave and the force that can be felt as an explosive blast.

This mechanism is fundamental to understanding how explosions operate since it is the rapid transformation of a solid or liquid into gas, coupled with a significant release of energy, that propels debris and creates the explosive effect. Other options like a sudden mechanical reaction or the movement of solid materials might contribute to an explosion's hazards or dynamics but do not directly explain the primary source of force, which is the quickly expanding gases. Meanwhile, long-lasting pressure buildup suggests a more gradual process rather than the rapid and explosive nature of gas expansion.