In physisorption adsorbent does not show specificity for any particular gas because ______________.

Physisorption is reversible in nature, because desorption of gas takes place either by increasing the temperature or decreasing the pressure. So option (iv) is wrong.

Physisorption does not depend upon the nature of gas, whether ideal or real. So (ii) is not true.

Since physisorption is an exothermic process, it has low enthalpy( 20-40 KJ/mol) in general. Therefore, option (iii) is not correct.

In physisorption, adsorbent and adsorbate get attached with the help of vander waals forces, which is irrespective of the nature of the gas. So (i) is the correct option.