The correct answer is B) volume of cooling water flow. The differential temperature of the main condenser cooling water is primarily affected by the volume of cooling water flow. As the volume of cooling water flow decreases, the temperature difference between the inlet and outlet of the condenser will increase, as there is less water available to absorb the same amount of heat from the condensing steam. Conversely, an increase in the volume of cooling water flow will result in a lower differential temperature, as more water is available to absorb the heat. The other options are incorrect because they do not directly impact the differential temperature of the main condenser cooling water. Condensate pump pressure, boiler feed pump pressure, and sea temperature can influence other aspects of the steam plant operation, but they do not have a direct effect on the cooling water temperature differential.