Since, adsorption involves electron transfer from metal to O₂, it is chemical adsorption not physical adsorption, hence (a) is incorrect. Adsorption is spontaneous which involves some bonding between adsorbent and adsorbate, hence exothermic. The last occupied molecular orbital in O₂ is $$\pi$$*2p. Hence, electron transfer from metal to oxygen will increase occupancy of $$\pi$$*2p molecular orbitals. Also increase in occupancy of $$\pi$$*2p orbitals will decrease bond order and hence increase bond length of O₂.