Which condition is critical for an explosion to occur?

For an explosion to occur, a correct fuel-oxidizer mixture is critical. Combustion, which leads to an explosion, requires an appropriate balance between fuel and oxidizing agent. In most cases, the oxidizer is a gas like oxygen found in the air. If the fuel is too rich or too lean compared to the amount of oxidizer, the combustion process may not happen effectively, or in some instances, it may not occur at all.

When the fuel and oxidizer are in a precise proportion, it allows for a rapid chemical reaction to take place, liberating energy in the form of heat and pressure, which can result in an explosion. This mixture ratio is often represented by terms like the lean and rich mixtures, where a certain range has to be maintained for efficient combustion to progress towards an explosive reaction.

While the other conditions listed may influence the possibility of an explosion, such as having a combustible liquid or the specific temperature, they are not inherently critical for an explosion to occur as the correct mixture is. Moreover, the absence of air typically would prevent combustion from initiating due to lack of oxygen, making it also less relevant in assuring an explosion can happen.