How does 'auto-ignition' occur?

Auto-ignition refers to the process where a substance spontaneously ignites due to heat generated internally, rather than from an external ignition source like a flame or spark. In this context, fuel that is chemically heated reaches its ignition temperature due to exothermic reactions that occur during degradation or decomposition. Essentially, the temperature of the fuel increases because of the chemical reaction itself, leading to combustion without the need for any external ignition source.

This phenomenon is particularly relevant in scenarios involving volatile fuels or substances that create sufficient heat from their own chemical processes, making option B the most accurate choice in understanding auto-ignition. The other responses describe different processes of ignition that involve external sources or different types of reactions, which do not accurately represent the essence of auto-ignition.