Which of the following best describes a "boiler"?

A boiler is defined as a closed heating appliance specifically designed to generate hot water or steam for various heating applications, including space heating, domestic hot water, and even industrial processes. The key aspects of a boiler include its function of heating water or producing steam under pressure, as well as its closed system design which allows for efficient heating and circulation.

In contrast to the other options, a boiler is not primarily intended for cooking food, which is the role of devices like ovens or stoves. Additionally, it differs from open flame cooking devices, which are specifically designed for culinary purposes rather than heating water or producing steam. Finally, while a water storage tank may hold hot water, it does not involve the process of heating or generating steam, which is central to the definition of a boiler. Therefore, option B accurately captures the essential characteristics of what a boiler is and how it operates within mechanical systems.