The concept of overall coefficient of heat transfer is used in case of heat transfer by

Answer: Option D

It is used in calculating the heat transfer, typically by convection or phase transition between a fluid and a solid.
The heat transfer coefficient has SI units in watts per squared meter kelvin: $$\frac{\text{W}}{{\text{m}}^2\text{K}}$$ .
The heat transfer coefficient is the reciprocal of thermal insulance.