The reading of a combustible gas indicator indicates the percentage of the __________.

Correct Answer: Option B — lower explosive limit of a flammable gas concentration

Combustible gas indicators are calibrated to measure the concentration of flammable gases relative to the Lower Explosive Limit (LEL). The LEL is the lowest concentration of vapor in air capable of producing a flash of fire. Monitoring this ensures the atmosphere remains below the threshold where ignition becomes possible.