The correct answer is (b) Mass transfer → Diffusion → Adsorption → Reaction → Desorption → Diffusion → Mass transfer
Easiest explanation: The catalytic reaction occurs by the mass transfer to the catalytic surface, followed by the diffusion of the reactant gas through the product layer. Reaction occurs at the gas – catalyst interface. Diffusion of the product gas occurs through the product layer. Mass transfer of the product gas occurs back through the gas film.