Which statement is true regarding the definition of a boiler?

The correct answer states that a boiler is an open vessel transforming water into steam under pressure. This definition is accurate as it encompasses the primary function of a boiler, which is to convert water into steam through the application of heat while maintaining the ability to contain pressure. In the context of boiler operations, the process typically takes place in a system designed to manage the heat provided by fuel combustion or other heat sources, often leading to steam formation that can be used for various applications, such as heating or power generation.

The notion of an open vessel suggests a certain type of boiler where water can be exposed to the atmosphere while it undergoes transformation due to heat application. This aligns correctly with how some boilers operate, particularly in processes where steam is produced on demand.

The other statements misinterpret aspects of a boiler’s function or structure. For example, defining a boiler as solely used for steam generation without heat application overlooks the fundamental mechanics of how boilers operate. Heat application is essential for steam generation. Similarly, the references to vessels being open or closed and to the pressure dynamics are not accurately depicted, further clarifying why the correct answer stands out in its description of a boiler's essential characteristics.