The correct answer is D) spontaneous combustion. Spontaneous combustion refers to the process where heat is generated by a chemical reaction within a substance, and this heat builds up to the point of ignition, causing the substance to catch fire without an external ignition source. This is in contrast to other forms of ignition, such as chemical ignition or radiation ignition, which require an external source of heat or energy to initiate the combustion process. The other answer choices are incorrect because they do not accurately describe the self-heating and self-ignition process that is the defining characteristic of spontaneous combustion.