The correct answer is A) wind box. In a combustion control system, the air flow transmitter receives its high-pressure signal from the wind box, which is the chamber that distributes the air supply to the boiler furnace. The wind box is responsible for regulating the air flow to the boiler, and the air flow transmitter monitors this pressure to ensure proper combustion control. The other options are incorrect because the furnace (B) does not directly supply the high-pressure signal, the fan discharge (C) is downstream of the wind box, and the smoke box (D) is not the source of the air flow signal.