The common unit of heat is the Btu, British thermal unit. What units of mass (e.g., grams, ounces, etc.) and temperature (oC, oF) are given for the definition of this heat unit? Mass unit and the temperature unit

Pounds, degrees Fahrenheit.
A British thermal unit, or Btu, is defined as the amount of heat needed to raise the temperature of ##”1 lb”## of water by ##1^@”F”##.
This means that if you have a ##”1-lb”## sample of water at a temperature of, say, ##77^@”F”##, you would need to supply it with ##”1 Btu”## of heat in order to get its temperature to increase to ##78^@”F”##.