Answer: Option (c) is the correct answer.
Step-by-step explanation:
The difference in temperature when per unit of heat flux is required by one unit of heat flux between a barrier of warmer and colder surface present in steady state condition is known as R-value.
Mathematically, R-value =
Thermal resistance of insulating products or materials can be known with the help of R-value. As more is the r-value of a substance more will be its insulating power.
Therefore, we can conclude that insulating materials are rated by their r-value.