The correct answer is B) hydrocarbon content of the atmosphere is less than the upper explosive limit. The reason this is the correct answer is that a combustible gas indicator relies on the presence of combustible gases in the atmosphere to function properly. If the hydrocarbon content of the atmosphere is below the upper explosive limit, the indicator will not be able to detect the presence of those gases and will either become inoperative or provide erroneous readings. The other options are incorrect because the presence of CO2 (option A) or inerted content (option C) would not directly affect the operation of the combustible gas indicator, and the presence of sufficient oxygen (option D) is not the key factor in the indicator's functionality.