The correct answer is C) efficient combustion. The absence of carbon monoxide (CO) in the flue gas of a boiler indicates that the combustion process is efficient, meaning the fuel is being burned completely. Efficient combustion occurs when there is the proper air-to-fuel ratio, allowing the fuel to be fully oxidized and producing minimal CO as a byproduct. The other options are incorrect because: A) Contaminated fuel oil would more likely result in the presence of CO due to incomplete combustion. B) Low carbon content of the fuel would not necessarily indicate efficient combustion. D) Insufficient air would lead to incomplete combustion and the presence of CO in the flue gas.