What is defined as any substance capable of undergoing combustion?

The term that defines any substance capable of undergoing combustion is "fuel." Fuels are materials that can react with an oxidizer, typically oxygen in the air, to produce heat and often light in the process of combustion. Fuels can vary widely, including solids like wood or coal, liquids such as gasoline or oil, and gases like propane or natural gas.

Understanding how fuels interact with heat and oxygen is fundamental to fire investigation. All combustion processes rely on the presence of fuel, as it not only supports the reaction but also determines the characteristics of the fire, such as intensity and duration.

While "combustible material" might seem similar, it typically refers to substances that can burn rather than the broader category of fuels that specifically support combustion. Likewise, an "ignition source" refers to something that initiates the combustion process, and a "propellant" is often associated with materials that generate thrust or create pressure, which may not necessarily involve combustion. Thus, the defining characteristic of fuel focuses on its ability to undergo combustion, making it the correct answer.