The correct answer is D) Main condenser. The main condenser of a steam plant is typically cooled by seawater. This is due to the need for a reliable and abundant source of cooling fluid to condense the steam exiting the turbine and maintain the vacuum required for efficient plant operation. The other condensers listed, such as the gland exhaust condenser, air ejector condenser, and vent condenser, are generally cooled by the plant's freshwater cooling system rather than seawater.