The percentage of a flammable substance in air that will burn when in contact with an ignition source is:

• A. ignition temperature.
• B. boiling point.
• C. flammable-explosive range.
• D. flash point.

Answer: (C)

Understanding how a fuel–air mixture behaves helps explain why this term fits. A flammable vapor in air will only ignite if its concentration lies within a certain band called the flammable-explosive range. This range is defined by two limits: the lower explosive limit (LEL), below which the mixture is too lean to burn, and the upper explosive limit (UEL), above which it’s too rich to burn. If the mixture sits within that range and there’s an ignition source, ignition can occur. Temperature-related terms like ignition temperature and flash point describe when ignition can happen due to heat, not the proportion of fuel in air, and the boiling point is unrelated to ignition. So the concept describing the percentage in air that will burn when an ignition source is present is the flammable-explosive range.