Ignition temperature is defined as:

• A. Minimum temperature at which a fuel will evaporate
• B. Minimum temperature at which a fuel will ignite with external flame
• C. Minimum temperature at which a fuel will begin to chemically react
• D. Minimum temperature at which self-sustained combustion occurs independent of an ignition source

Answer: (D)

The key idea is autoignition—the temperature at which a fuel will start to burn and sustain combustion on its own, without an external flame or spark. This is what ignition temperature refers to in safety and fire science: the point where self-sustained combustion can occur independently of an ignition source. It’s different from a flash point, which is the lowest temperature at which enough vapor can form to ignite in the presence of a flame or spark; that still requires an external heat source. It’s also not simply the temperature at which evaporation or general chemical reaction begins. So the correct concept is the minimum temperature at which self-sustained combustion occurs without an ignition source.