Nusselt number is a dimensionless number used in heat transfer to characterize the convective heat transfer process. It relates the convective heat transfer to the conductive heat transfer and is defined as the ratio of convective heat transfer to conductive heat transfer across a boundary layer.

The Nusselt number in the case of forced convection is influenced by two main factors: the Reynolds number and the Prandtl number.

a) Reynolds number and Prandtl number:
The Reynolds number (Re) is a dimensionless number that represents the ratio of inertial forces to viscous forces in a fluid flow. It is defined as the product of the fluid velocity, characteristic length, and fluid density divided by the fluid viscosity.

The Prandtl number (Pr) is a dimensionless number that represents the ratio of momentum diffusivity to thermal diffusivity in a fluid. It is defined as the ratio of the dynamic viscosity to the thermal conductivity of the fluid.

The Nusselt number (Nu) in the case of forced convection is given by the following equation:

Nu = f(Re, Pr)

where f is a function that relates the Nusselt number to the Reynolds number and the Prandtl number. The exact form of the function depends on the specific flow conditions and geometry.

b) Reynolds number only:
If the Nusselt number in the case of forced convection was solely a function of the Reynolds number, it would imply that the Prandtl number has no effect on the convective heat transfer process. However, this is not the case, as the Prandtl number plays a crucial role in characterizing the fluid properties and their influence on heat transfer.

c) Grashoff number only:
The Grashoff number (Gr) is a dimensionless number that represents the ratio of buoyancy forces to viscous forces in a fluid flow. It is defined as the product of the characteristic length, fluid density, gravitational acceleration, and the temperature difference divided by the fluid viscosity squared.

The Grashoff number is not directly related to the Nusselt number in the case of forced convection. It is primarily used to characterize natural convection processes, where buoyancy forces dominate.

d) Grashoff number and Prandtl number:
The Grashoff number and Prandtl number are not the only factors influencing the Nusselt number in the case of forced convection. While the Prandtl number affects the convective heat transfer due to its influence on the fluid's thermal properties, the Grashoff number is not directly related to forced convection.

In summary, the correct answer is option 'A' as the Nusselt number in the case of forced convection is a function of both the Reynolds number and the Prandtl number.