When is a substance considered flammable in terms of its vapor presence?

A substance is considered flammable when it is at its flash point, which is the minimum temperature at which it can form an ignitable mixture with air. At this temperature, the vapor produced by the substance is sufficient to ignite if there is an ignition source nearby. Understanding the flash point is crucial for assessing fire hazards, as it indicates the point at which the vapors can contribute to combustion. Monitoring temperatures around the flash point helps in ensuring safe handling and storage of flammable materials.

The other options relate to different temperature thresholds but do not adequately define flammability. Being below the boiling point does not guarantee that a substance is flammable, as it may not be producing enough vapor. Conversely, being above a hypothetical combustion point isn't a standard measure used to determine flammability, and reaching a solid state generally indicates a lack of produce vapor required for ignition.