Which of the following types of boilers primarily uses water to transfer heat?

The water-tube boiler is primarily designed to use water as the medium for transferring heat. In a water-tube boiler, water circulates in tubes that are heated externally by the products of combustion. This design allows for efficient heat transfer because the water is in direct contact with the heat source, typically producing steam that can be used for power generation or heating. The arrangement allows for higher pressures and temperatures than other boiler designs.

Other types of boilers can also utilize water for heat transfer, but their primary design focus varies. Electric boilers, for example, use electric heating elements to heat water, while fire-tube boilers rely on hot gases passing through tubes immersed in water to impart heat. The question specifically emphasizes the dominant mechanism, which in the case of the water-tube boiler, is the direct and efficient use of water to transfer heat, making it the most appropriate choice.