Which of the following best describes a fire-tube boiler?

A fire-tube boiler is characterized by a design in which water is contained within large tubes that are surrounded by hot gases produced from burning fuel. This setup is advantageous because the hot gases transfer their heat to the water in the tubes, leading to efficient steam generation. The tubes allow for effective heat exchange, as the large surface area facilitates quicker evaporation of water into steam.

In contrast, other options describe different types of boilers or methods of heating. For instance, electric boilers use heating elements submerged in water to generate steam, which is a fundamentally different approach from how fire-tube boilers operate. The description of the furnace being located inside the water chamber aligns more with water-tube boilers, where the water flows through tubes in a furnace environment. Lastly, the burning of solid fuels exclusively does not accurately reflect the operational capabilities of fire-tube boilers, as they can utilize various fuel types, including gas and oil, not just solid fuels. Thus, the best descriptor of a fire-tube boiler remains the method of hot gases surrounding water-filled tubes.