What does autoignition refer to?

Autoignition refers to the phenomenon where a substance ignites spontaneously due to being heated to its ignition temperature without the need for an external ignition source such as a spark or flame. This process occurs when the temperature of a material reaches a point where combustion can occur due to thermal energy alone. In many instances, this can happen with materials that have been exposed to elevated temperatures for an extended period, potential ignition sources, or specific environmental conditions.

In this context, the other choices describe different ignition mechanisms that involve an external influence. Combustion initiated by flames relies on an existing flame source, while purposeful ignition using matches indicates a deliberate act to ignite a fire with an external spark. The controlled burning of materials denotes a regulated process in which combustion is intentionally managed, typically for safety or efficiency, rather than an uncontrolled or spontaneous event. Thus, the specific nature of autoignition aligns clearly with the notion of spontaneous ignition due to accumulated heat, making the selected answer accurate.