Following are the events taking place to explain adsorption theory 
I : Desorption
II : Diffusion of the reactants along the surface 
III : Adsorption of the reactants
IV : Formation of the activated surface complex  These events are taking place in the following order. 

During adsorption, first there is diffusion of reactant molecules on the surface of adsorbent. Then, adsorption of reactant molecules on the surface of adsorbent takes place. Later, their is formation of an activated complex due to reaction between absorbent and reactant molecules. After, formation of products, desorption process is taken place.

Hence, option-2 is the correct option.