The correct answer is B) condensing of the exhausting steam. The vacuum in the main condenser is primarily caused by the condensation of the exhausting steam. As the steam enters the condenser, it comes into contact with the cooling water, which causes the steam to condense into water. This condensation process creates a partial vacuum in the condenser, which helps to draw more steam into the condenser, further enhancing the vacuum. The other options are incorrect because they do not directly contribute to the primary cause of the vacuum in the main condenser. The main air ejector helps to remove air and non-condensable gases from the condenser, the condensate pump removes the condensed water, and the after condenser loop seal helps to maintain the vacuum in the condenser, but they are not the primary drivers of the vacuum formation.