ANS:C - Schmidt number.

The dimensionless group in mass transfer that is equivalent to the Prandtl number in heat transfer is the Schmidt number. Schmidt number (Sc) is defined as the ratio of momentum diffusivity (kinematic viscosity) to mass diffusivity. It represents the ratio of the momentum boundary layer thickness to the concentration boundary layer thickness in mass transfer, just as the Prandtl number represents the ratio of the thermal boundary layer thickness to the momentum boundary layer thickness in heat transfer. Therefore, the correct answer is: Schmidt number