The correct answer is A) High pressure bleed steam. A third stage feed heater, if fitted, receives its source of heat from high pressure bleed steam taken from the turbine. This high pressure bleed steam is used to heat the boiler feed water before it enters the boiler, improving the overall efficiency of the steam system. The other options are incorrect because: B) Auxiliary exhaust steam is not used for feed heating, C) Low pressure bleed steam would not provide enough heat, and D) Intermediate pressure bleed steam could be used, but high pressure bleed steam is the more common and efficient choice.