What defines spontaneous combustion?

Spontaneous combustion refers to the process where a material ignites due to heat generated internally without an external flame or ignition source. This phenomenon occurs when a substance undergoes a slow oxidation process that generates heat. If this heat is not dissipated, it can build up to the point where the temperature reaches the ignition point of the material itself, leading to combustion.

In the context of this question, the correct choice highlights the key component of spontaneous combustion — the generation of heat through internal chemical action. This process can occur in various materials, such as oily rags or compost piles, where the heat from microbial activity or oxidation can accumulate and eventually lead to ignition.

In contrast, other options describe different ignition processes: external flames or sparks (as in the first choice), the ignition of vapors that are already airborne (as in the third choice), and rapid oxidation that results in a flash fire (as in the fourth choice). These scenarios do not involve the internal heating and subsequent ignition characteristic of spontaneous combustion.