Which of the following types of boilers is an example of an internally fired boiler?

The correct answer is the Scotch Marine boiler because it is designed with the fire contained within its tubes, which are submerged in water. This design allows for efficient heat transfer as the hot gases pass through the fire tubes, heating the water and producing steam. Internally fired boilers have their heating elements inside the boiler itself, allowing for a more compact design and enhanced thermal efficiency.

In contrast, other types like the horizontal return tube boiler, crown sheet boiler, and vertical fire tube boiler have different configurations or principles that do not classify them as internally fired. For instance, horizontal return tube boilers typically have an external firebox, and while crown sheet boilers utilize a different design for steam generation, they can also feature external fire elements. The vertical fire tube boiler, while it efficiently heats water, does not fit the definition of internally fired because its heat can primarily originate from an external source.