Upper Explosive Limit (UEL) is defined as:

• A. the minimum vapor concentration below which a flame cannot propagate
• B. the maximum vapor to air concentration above which a flame will not propagate
• C. the minimum temperature for ignition
• D. the self-ignition temperature

Answer: (B)

The main idea is where the fuel–air mixture can still support flame propagation. For vapors in air, there’s a flammable range bounded by a lower limit (too lean to ignite) and an upper limit. The upper explosive limit is the highest vapor-to-air concentration at which a flame can continue to propagate. If the concentration exceeds this limit, the mixture is too rich in fuel to sustain combustion—the flame cannot be propagated because there isn’t enough oxygen relative to the fuel. The ignition temperature and self-ignition temperature describe heat/energy needed to start combustion, not the concentration limits for flame propagation, so they don’t define the upper limit.