The lowest temperature at which a substance catches fire is called its:

ignition temperature

The correct answer is option 4. ignition temperature.

The ignition temperature (or kindling point) is the minimum temperature at which a substance will ignite and sustain combustion in the presence of an external ignition source, such as an open flame, spark, or heat. It is the temperature at which the substance's vapor or gas phase can undergo a combustion reaction with oxygen in the air.

When a substance is heated to its ignition temperature, the molecules gain sufficient energy to initiate and maintain the combustion process. Below this temperature, the substance may not catch fire, but once the ignition temperature is reached, the combustion reaction becomes self-sustaining.

The ignition temperature is a crucial factor in understanding the fire safety of materials and substances. It varies for different materials, depending on their chemical composition and properties. Some substances have lower ignition temperatures and are more prone to catching fire, while others require higher temperatures for ignition.