What is required for a fire to start with gaseous fuels?

For a fire to start with gaseous fuels, the presence of an appropriate ignition source is essential. Gaseous fuels require a specific temperature to ignite, but that temperature alone does not suffice if an ignition source is not present. An ignition source provides the necessary energy to initiate the combustion process. This could be a spark, flame, or other heat sources that can raise the fuel to its ignition temperature.

Other options, while related to the combustion process, do not necessarily need to be present for ignition to occur. A high combustion temperature is important for sustaining the fire, but it does not start the process itself without an ignition source. A catalyst may facilitate combustion, but it is not a requirement for all gaseous fuels. The presence of a liquid phase is irrelevant concerning gaseous fuels, as ignition can occur in the gaseous state without the necessity of transitioning through a liquid phase. Thus, the correct choice highlights the crucial role of an ignition source in the fire-starting process for gaseous fuels, as it is fundamentally necessary to initiate combustion.