The correct answer is B) ensure the condensation of air ejector steam. When a steam turbine is warming up, the main condenser needs to maintain a high vacuum to ensure efficient operation. Recirculating the condensate back to the main condenser helps remove any non-condensable gases, such as air, that could accumulate and degrade the vacuum. This process of condensing the air ejector steam is crucial for maintaining the desired vacuum levels in the condenser, which is essential for the overall performance and efficiency of the steam turbine system. The other answer choices are incorrect because: A) Cooling the main condenser shell is not the primary purpose of recirculating the condensate during the warm-up process. C) This is the correct answer, so no further explanation is needed. D) Maintaining the DC heater water level is not directly related to the recirculation of condensate to the main condenser during turbine warm-up.