The correct answer is A) Economizer drain valve. Before lighting off a boiler, the economizer drain valve should be closed to prevent water from draining out during the heating process. The economizer is a heat exchanger that preheats the boiler feedwater, and the drain valve allows for the removal of any accumulated sediment or water from the economizer. Keeping this valve closed is important to maintain proper water levels and prevent damage to the boiler system during the boiler lighting procedure. The other options are incorrect because the air cock valve (B) is used for releasing air from the boiler, the superheater vent valve (C) is used for venting the superheater, and the superheater drain valve (D) is used for draining the superheater, none of which are necessary to close before lighting off the boiler.