The correct answer is A) Light brown haze. A light brown haze from the stack indicates the most complete combustion and highest boiler efficiency. This is because complete combustion results in minimal unburnt fuel particles, leading to a light-colored, almost transparent exhaust plume. In contrast, incomplete combustion produces byproducts like soot and unburnt fuel, resulting in a darker, more opaque stack appearance. B, C, and D are incorrect because they indicate incomplete combustion and lower boiler efficiency. A clear stack could mean the boiler is not operating, white smoke suggests incomplete vaporization of fuel, and black smoke indicates excessive unburnt fuel particles.