The correct answer is D) more heat is liberated by the production of CO2 than CO. When burning fuel oil in a boiler, a high CO2 content in the stack gas is desired because it indicates efficient combustion and the production of CO2 rather than CO. This is because the complete combustion of fuel to CO2 releases more heat energy compared to the partial combustion to CO. The formation of CO2 is the desired outcome, as it maximizes the heat liberated from the fuel. The other options are incorrect because: A) While efficient combustion is indicated by high CO2, the heat liberated is actually greater than the heat produced by burning to CO. B) The heat liberated is greater when producing CO2 compared to CO. C) Less excess air is required to produce CO2 than CO, but this is not the primary reason for the desired high CO2 content.