Which of the following types of boilers is a package boiler?

A package boiler is a pre-assembled system that comes as a complete unit, ready for installation and use. It integrates various components needed for steam or hot water generation, such as the boiler shell, burner, and controls, all in one package, which simplifies installation and reduces the space needed on-site.

A cast iron package boiler fits this definition perfectly. It is designed as a compact, portable solution that can be easily transported and installed without requiring extensive on-site assembly. These types of boilers typically provide high efficiency and are widely used in residential and light commercial applications.

Components not categorized as package boilers, such as HRT boilers, vertical fire tube boilers, or locomotive boilers, typically feature configurations that require individual assembly of components on-site or don't fit the compact, pre-packaged design common in package boilers. Therefore, recognizing the defining characteristics of a package boiler leads to the understanding that a cast iron package boiler is indeed the correct answer to this question.