How can the speed of solvent molecules be slowed down?

Explanation:

When we decrease the temperature, then there will be less collision between the particles. As a result, there will be decrease in the kinetic energy of molecules. Therefore, molecules will start to settle at the bottom.

Hence, there will be decrease in the speed of molecules.

Thus, we can conclude that on decreasing the temperature the speed of solvent molecules be slowed down.

By decreasing the temperature of the solvent

Explanation:

The speed of the solvent is dependent on the kinetic energies of the solvent molecules. The higher kinetic energies means that there is a diffusion of the molecules in solution. Higher temperatures mean higher kinetic energies of the molecules. Thus, by definition, temperature is the measure if the average kinetic energies of molecules.

Lowering the temperatures lowers the average kinetic energies if the molecules. Hence less diffusion.

the answer is definitely c.