A British Thermal Unit (BTU) is defined as the amount of heat required to:

A British Thermal Unit (BTU) is officially defined as the amount of heat needed to raise the temperature of one pound of water by one degree Fahrenheit. This specific definition is rooted in fundamental principles of thermodynamics and provides a clear and consistent measurement for energy in heating applications.

The BTU is critical for understanding energy transfer in heating systems, and since it pertains specifically to water, a substance with a high specific heat, it serves as a useful baseline for assessing the energy requirements of various heating processes. In practical applications, knowing that raising the temperature of one pound of water requires a specific amount of energy allows technicians to calculate the necessary energy input for heating systems, ensuring they operate efficiently and effectively.

While the other answer choices refer to variations of water or air, they do not align with the precise definition and understanding of a BTU in standard thermodynamic terms.