Which of the following is considered an ignition source in vehicles?

An ignition source in vehicles refers to any material or condition that has the potential to ignite a flammable material, which can lead to a fire. Open flames are a clear and common ignition source because they generate sufficient heat to ignite combustible materials found in or around vehicles. In the context of vehicles, sources like lit cigarettes, matches, or any form of fire can easily ignite fuel, oil, or other flammable substances found in cars.

Cold temperatures, low humidity, and environmental pollution do not directly cause ignition. While they can influence the likelihood of a fire occurring or the conditions of the combustion process, they are not sources of ignition themselves. For example, cold temperatures may affect the volatility of fuels, but they do not ignite materials on their own. Similarly, low humidity may reduce the moisture content in materials, which can impact fire behavior, but it does not provide an ignition source. Environmental pollution, although it may introduce additional flammable materials, does not serve as a direct ignition source.

Therefore, among the options provided, open flames stand out as a clear ignition source due to their capability to directly initiate combustion in the presence of flammable materials typically found in vehicles.