The Fourier number (defined as a.t/L²) is used in the analysis of problem involving heat transfer by

ANS:C - transient conduction

The Fourier number, defined as 2αt​, where α is the thermal diffusivity, t is time, and L is a characteristic length, is primarily used in the analysis of problems involving transient conduction. Transient conduction refers to heat transfer situations where the temperature within a solid material varies with time. The Fourier number allows us to quantify the ratio of the rate of heat conduction to the rate of heat storage within the material over time. So, the correct answer is: "transient conduction"