The statement(s) that correctly defines the rate of diffusion is-

                I.          Rate of diffusion is the speed at which particles of two matter intermix with each other.

                II.          Rate of diffusion increases with an increase in kinetic energy.

               III.          Rate of diffusion decreases with an increase in temperature.

               IV.          Rate of diffusion is inversely proportional to the mass of the particles.

Diffusion is the mixing of particles of two types of matter. For example, when salt is added to water, the particles of salt diffuse into the particles of water.

Thus, rate of diffusion is a term that tells how faster the two particles are mixing. As diffusion is based on the moving nature of particles, thus kinetic energy plays an important role in diffusion.

The higher the kinetic energy, the more the speed of particles, and thus higher is the rate of diffusion.

Hence, statements I and II are correct.