The correct answer is A) Closing the water seal line to the packing gland. Closing the water seal line to the packing gland can prevent a centrifugal condensate pump from developing its rated capacity. The water seal line is designed to prevent air from entering the pump suction, which could cause cavitation and reduce the pump's performance. By closing this line, air would be allowed to enter the pump, disrupting the pump's ability to develop its full rated capacity. The other options are incorrect because: B) Operating the pump with a positive suction head would not prevent the pump from developing its rated capacity; C) Venting the pump to the vacuum side of the condenser would not affect the pump's capacity; and D) Flooding of the main condenser hotwell would not directly impact the condensate pump's performance.