The correct answer is C) going up the stack. In an oil-fired boiler, the major heat loss is through the hot exhaust gases that go up the stack. This is because a significant portion of the heat generated by the combustion of the fuel is lost through the stack, rather than being effectively transferred to the water or steam in the boiler. The heat lost through the stack represents energy that could have been used to generate steam or heat, but is instead discharged to the atmosphere. The other options are incorrect because they do not represent the major heat loss in an oil-fired boiler. Option A is incorrect because the heat required to change water into steam is necessary for the boiler's operation and is not considered a major heat loss. Option B is incorrect because the heat passing through the boiler casing is generally minimized through proper insulation. Option D is incorrect because the heat used in the economizer and air heater is intended to recover and reuse some of the waste heat, rather than representing a major heat loss.