The process of ignition causes pyrolysis in solid fuels and what in liquid fuels?

The correct answer is vaporization. In the context of liquid fuels, the process of ignition initiates the transition of the fuel from its liquid state to a vapor state. This is crucial because liquid fuels cannot burn in their liquid form; they must be vaporized first to produce a combustible mixture with air.

When heat is applied to a liquid fuel, it causes the molecules within the liquid to gain energy, which leads to vaporization. This vapor subsequently mixes with air and can ignite, sustaining combustion. This process is essential for understanding fire behavior, as it explains how liquid fuels contribute to fire development.

Other processes listed, such as thermal expansion, combustion, and ignition, do not directly describe the state change necessary for liquid fuels to ignite. Thermal expansion may occur in materials due to heat but is not relevant to the ignition process itself. Combustion refers to the chemical reaction of burning, which depends on the availability of vaporized fuel, and ignition is the initial spark or heat that starts burning, but again, it is vaporization that specifically defines how liquid fuels transition into a flammable state.