As per molecular orbital theory the pair of molecules which do not exist is

\begin{array}{|l|c|c|c|c|} \hline Molecules & \begin{array}{c} Total no. \\ of I \end{array} & \begin{array}{c} e in bonding \\ orbitals \end{array} & \begin{array}{c} e in anti- \\ bonding \\ orbital \end{array} & \begin{array}{c} Bond \\ order \end{array} \\ \hline (a) \mathrm{He}_2 & 4 & 2 & 2 & 0 \\ (b) \mathrm{Be}_2 & 8 & 4 & 4 & 0 \\ (c) \mathrm{Li}_2 & 6 & 4 & 2 & 1 \\ (d) \mathrm{B}_2 & 10 & 6 & 4 & 1 \\ (e) \mathrm{C}_2 & 12 & 8 & 4 & 2 \\ (f) \mathrm{Nee}_2 & 20 & 10 & 10 & 0 \\ \hline \end{array}