What initiates combustion in a material due to sufficient internal heat caused by a chemical reaction?

The correct answer is spontaneous ignition, which refers to the process where a material ignites without an external ignition source, due to sufficient internal heat generated from a chemical reaction occurring within the material itself. This internal reaction can raise the temperature of the material to its ignition point, resulting in combustion.

In the context of combustion, spontaneous ignition is significant because it highlights the potential fire hazards that can arise from certain materials undergoing exothermic reactions. Often, this occurs in cases involving organic materials that decompose, such as oily rags, compost piles, or other substances that can generate heat as they break down chemically.

The other options reflect different fire-related phenomena: combustion burst refers to a rapid and often violent release of energy during combustion typically initiated by an external source; autoignition is similar to spontaneous ignition but usually involves combustion occurring at a specific temperature without an open flame; and flame propagation describes the spread of an existing flame through a combustible material. While these concepts are related to the overall understanding of fire behavior, they do not directly capture the self-initiated aspect of combustion described in spontaneous ignition.