Correct option is C
Molecular orbitals are classified as σ, π, or δ according to their rotational symmetry about the internuclear axis, and (in centrosymmetric species) as g or u according to their symmetry with respect to inversion.
σ orbitals are formed by allowing overlap between atomic orbitals that have cylindrical symmetry around the internuclear axis, which is conventionally labelled z. The notation σ signifies that the orbital has cylindrical symmetry; atomic orbitals that can form σ orbitals include the 2s and 2pz orbitals on the two atoms. From these four orbitals (the 2s and the 2pz orbitals on atom A and the corresponding orbitals on atom B) with cylindrical symmetry we can construct four σ molecular orbitals, two of which arise predominantly from interaction of the 2s orbitals, and two from interaction of the 2pz orbitals. These molecular orbitals are labelled 1σg, 1σu, 2σg, and 2σu, respectively. Interaction between a 2s on one atom and a 2pz orbital on the other atom is possible if their relative energies are similar.
The remaining two 2p orbitals on each atom, which have a nodal plane containing the z-axis, overlap to give π orbitals. Bonding and antibonding π orbitals can be formed from the mutual overlap of the two 2px orbitals, and also from the mutual overlap of the two 2py orbitals. This pattern of overlap gives rise to the two pairs of doubly degenerate energy levels (two energy levels of the same energy) and labelled 1πu and 1πg.
For homonuclear diatomic, it is sometimes convenient to signify the symmetry of the molecular orbitals with respect to their behaviour under inversion through the centre of the molecule. The operation of inversion consists of starting at an arbitrary point in the molecule, travelling in a straight line to the centre of the molecule, and then continuing an equal distance out on the other side of the centre. The orbital is designated g (for gerade, even) if it is identical under inversion, and u (for ungerade, odd) if it changes sign. Thus, a bonding σ orbital is g and an antibonding σ orbital is u. On the other hand, a bonding π orbital is u and an antibonding π orbital is g.
