Prandtl number signifies the ratio of

- $\frac{Momentum \: Diffusivity}{Thermal \: Diffusivity}$
- $\frac{Mass \: Diffusivity}{Thermal \: Diffusivity}$
- $\frac{Thermal \: Diffusivity}{Momentum \: Diffusivity}$
- $\frac{Thermal \: Diffusivity}{Mass \: Diffusivity}$