a. ΔH < 0, ΔS < 0
b. ΔH < 0, ΔS > 0
c. ΔH > 0, ΔS > 0
d. ΔH = 0, ΔS > 0
During adsorption, there is always a decrease in residual forces of the surface, i.e.,
there is a decrease in surface energy which appears as heat. Adsorption, therefore, is
invariably an exothermic process, i.e., ΔH < 0. When a gas is adsorbed, the freedom
of movement of its molecules becomes restricted. This amounts to a decrease in the
entropy of the gas after adsorption, i.e., ΔS < 0 is negative.
Hence, the correct option is a.