What does the flash point of a liquid indicate?

The flash point of a liquid is defined as the lowest temperature at which it can form an ignitable mixture with air. This is significant because it indicates the point at which sufficient vapors are produced to be ignited by a spark or flame, leading to combustion. Understanding the flash point is crucial for fire safety as it helps in the classification of liquids regarding their flammability and potential hazards in various conditions.

The other options do not accurately describe the flash point. The temperature at which a substance burns refers more to the combustion temperature rather than the flash point. The term "combustion is irreversible" relates to the concept of ignition and has a different context from flash point. Lastly, spontaneous ignition refers to a material igniting without an external spark due to reaching a certain temperature under specific conditions, which is not what the flash point signifies. Thus, the defining characteristic of the flash point relates solely to the production of flammable vapors, affirming that the correct answer is indeed the lowest temperature for flammable vapor production.