ANS:D - molecular velocity and por radius of the catalyst.

The Knudsen diffusivity is dependent on the molecular mean free path only. Knudsen diffusivity arises in the Knudsen diffusion regime, which occurs when the characteristic length scale of the pore or channel is comparable to or smaller than the mean free path of the molecules. In this regime, molecular collisions with the pore walls dominate over collisions between molecules themselves. The mean free path is the average distance a molecule travels between collisions with other molecules. In the Knudsen diffusion regime, molecules spend more time traveling between collisions, leading to an enhanced diffusivity compared to bulk diffusion. Therefore, the Knudsen diffusivity is primarily dependent on the molecular mean free path, and it is not directly influenced by molecular velocity or the pore radius of the catalyst.