During change of ${\text{O}}_2$ to ${\text{O}}_2^{-}$ ion, the electron adds on which one of the following orbitals?

The molecular orbital configuration of ${\text{O}}_2$ ($16$ electrons) is:
$${\sigma }_{1s}^2{\sigma }_{1s}^{\ast 2}{\sigma }_{2s}^2{\sigma }_{2s}^{\ast 2}{\sigma }_{2p_z}^2({\pi }_{2p_x}^2={\pi }_{2p_y}^2)({\pi }_{2p_x}^{\ast 1}={\pi }_{2p_y}^{\ast 1})$$
The highest occupied molecular orbitals (HOMO) are the degenerate ${\pi }^{\ast }$ antibonding orbitals. When ${\text{O}}_2$ becomes ${\text{O}}_2^{-}$ (superoxide), the $17\text{th}$ electron enters one of these partially filled ${\pi }^{\ast }$ orbitals.