The correct answer is B) black stack smoke emission. Insufficient air for combustion in a boiler furnace will result in incomplete combustion, leading to the production of black smoke emissions from the stack. This is because without enough oxygen, the fuel cannot be fully burned, resulting in the release of unburnt carbon particles that create the black smoke. The other options are incorrect because: A) 0% carbon monoxide level would indicate complete combustion, which is the opposite of what occurs with insufficient air. C) White incandescent flame is associated with complete, efficient combustion, not incomplete combustion. D) High flame temperature is more likely to occur with excess air, not insufficient air for combustion.