The molecular orbital configuration of the given molecules is H2=σ1s2 (no electron anti-bonding) Li2=σ1s2σ∗1s2σ2s2 (two anti-bonding electrons) B2=σ1s2σ∗1s2σ2s2σ∗2s2 {π2py1=π2pz1} (4 anti-bonding electrons) Though the bond order of all the species are same (B.O=1) but stability is different. This is due to difference in the presence of no. of anti-bonding electron. Higher the no. of anti-bonding electron lower is the stability hence the correct order is H2>Li2>B2