Which of the following best defines a "fired boiler"?

A "fired boiler" refers specifically to a boiler that generates heat through the combustion of fuel. This process involves burning a fuel source—such as natural gas, oil, or coal—to produce hot gases, which then transfer their heat to water in the boiler, resulting in steam or hot water for heating purposes.

This definition is fundamental in understanding boiler types, as it highlights the importance of combustion in heating applications. Other options, while related to various boiler types, do not accurately describe what constitutes a fired boiler. For instance, a boiler that operates on electric power does not rely on combustion, and a nuclear boiler operates using heat generated from nuclear fission, which further distinguishes it from a fired boiler. Additionally, a boiler without any fuel source would not be capable of generating heat in the manner that defines a fired boiler, which is centered on the combustion process.