What is the definition of auto ignition temperature?

The definition of auto-ignition temperature refers to the lowest temperature at which a combustible material ignites in air without the need for an external spark or flame. This concept is crucial for understanding fire behavior and fire prevention measures.

When a material reaches its auto-ignition temperature, it undergoes thermal decomposition, leading to the release of flammable gases. If those gases are mixed with air and reach a high enough concentration, they can ignite spontaneously. This phenomenon makes auto-ignition temperature a critical factor in the handling, storage, and transportation of combustible materials, as it helps prevent unintentional fires.

The other options do not accurately capture this definition. For instance, the idea that combustion occurs with a spark or that it is a general temperature necessary for a fire to start does not consider the unique characteristics of auto-ignition temperatures specifically. Furthermore, the notion of all materials igniting simultaneously is not relevant, as auto-ignition temperatures vary greatly between different substances. Thus, option B stands out as the most precise and factual definition.