Answer:
A. British thermal units
Step-by-step explanation:
Now firstly we have to understand the definition of British thermal unit or BTU. It is defined as the amount of heat required to raise the temperature of 1 pound of water to 1 degree Fahrenheit. Now as expressed by its name, it is the unit for calculation of amount of heat. The most famous unit for heat is Joule.
1 BTU = 1055 Joules.
Now as we know, temperature is degree of hotness or coldness of body. So, for example, when on providing heat, when temperature of water raises from 50° F to 51° F, it will be stated that 1 BTU of heat is provided. Same concept can be applied for loss of heat.