In the crystal field of the complex ${\left[Fe\right(Cl\left)\right(CN{)}_4\left(O_2\right)]}^{4-}$, the electronic configuration of the metal is found to be $t_{2g}^6{e}_g^0$. Which of the following is true about this complex ion?

The correct options are
C Its IUPAC name will be chlorotetracyanosuperoxidoferrate(II) ion
D It is a diamagnetic complex
The electronic configuration of $Fe$ is given as $t_{2g}^6{e}_g^0$. This lets us conclude that $Fe$ is in +2 oxidation state. Balancing the charges, we see that the $O_2$ given is nothing but superoxide ligand. We know that the $O-O$ bond length in superoxide is longer than that in oxygen. ${\left[Fe\right(Cl\left)\right(CN{)}_4\left(O_2\right)]}^{4-}$ will be called chlorotetracyanosuperoxidoferrate(II) ion.
We also know that Cynaide is a strong field ligand and hence it will cause pairing in the $t_{2g}$ orbitals. This lets us conclude that the said complex will be diamagnetic in nature.