What is the minimum amount of gaseous fuel required for a mixture to be flammable or explosive?

The correct answer is the Lower Explosive Limit (LEL) because this term specifically defines the minimum concentration of a gaseous fuel in the air needed to create a flammable or explosive mixture. Below this concentration, the mixture is too "lean" to ignite, meaning there isn't enough fuel present to sustain combustion. The LEL is crucial for understanding the safety parameters required in environments where flammable gases may be present.

In contrast, the Upper Explosive Limit (UEL) refers to the maximum concentration of a combustible substance in air. Beyond this limit, the mixture becomes too "rich" to ignite. The term "Explosive Limit" is often used informally, but it does not specify whether it refers to the lower or upper end of the concentration range. Lastly, the flammable range encompasses both the LEL and UEL, illustrating the full spectrum of concentrations where combustion can occur, but does not specifically indicate the minimum concentration required for ignitability.