What describes a backdraft in explosions?

A backdraft is characterized by a rapid explosion of fire that occurs when oxygen, which has been previously depleted, is suddenly introduced into a situation where combustible gases have accumulated. This scenario typically arises in enclosed spaces where a fire has consumed most of the available oxygen, leading to smoldering combustion and the build-up of flammable gases. When a door or window is opened, or any other source that allows fresh air to enter is created, the influx of oxygen can ignite these gases almost instantaneously, resulting in an explosive fireball. This dramatic reaction is what makes option "B" the correct description of a backdraft.

The other options describe different phenomena. A slow ignition of vapors in air pertains to the initial stage of combustion and does not capture the explosive nature of a backdraft. A controlled release of gas pressure suggests a managed and safe handling of pressure, which is quite the opposite of the chaotic and uncontrolled nature of a backdraft. Lastly, a decrease in the fire's temperature does not relate to backdrafts, as a backdraft reaction typically involves an increase in temperature due to the sudden and violent combustion of previously trapped flammable gases.