The correct answer is A) The mixture is too rich to burn. When the concentration of flammable liquid vapors is above the upper explosive limit (UEL), the mixture is too rich with fuel, meaning there is too much fuel relative to the available oxygen. In this condition, the vapor-air mixture is not within the flammable range and cannot support combustion. The mixture is considered too rich to burn. The other options are incorrect because: B) Conditions are not perfect for combustion when above the UEL, C) The vapor is not about to explode in this condition, and D) The mixture is not too lean to burn, it is too rich.