Final answer:
The heat release rate is commonly expressed in kilowatts (kW), though British thermal units (Btu) per hour can also be used, especially within the United States. so, option A is the correct answer.
Step-by-step explanation:
The heat release rate is usually expressed in measurements such as kilowatts (kW), which is a unit of power representing the rate of energy transfer. In some contexts, particularly in the United States, you may encounter heat release rates expressed in British thermal units (Btu) per hour, which can be converted to watts for a standard comparison. For example, a household heater might be rated in Btu/hr, with 1 Btu equal to 1,055 joules and thus 1 Btu/hr equates to approximately 0.293 watts. However, when it comes to heat release rate directly related to the rate of energy transfer over time, kilowatts is the more commonly used unit.
Other energy units such as kilojoules per gram (kJ/g) might be used to express the energy content of substances such as food, determined using a bomb calorimeter. Similarly, the units of joules per gram or joules per mole are utilized when discussing the heat of vaporization. Nevertheless, for expressing the rate of heat release, kilowatts or kilowatts per meter squared (kW/m2) are preferred.