Choose the correct option.

Both ${\left[\mathrm{Co}{\left({\mathrm{NO}}_2\right)}_6\right]}^{4-}$ and ${\left[\mathrm{Co}{\left({\mathrm{NO}}_2\right)}_6\right]}^{3-}$involve ${\mathrm{d}}^2{\mathrm{sp}}^3$ hybridization, but the former is paramagnetic and the latter is diamagnetic.

As, one of the 3d electrons in ${\left[\mathrm{Co}{\left({\mathrm{NO}}_2\right)}_6\right]}^{4-}$ jumps to the 4d orbital, leaving the two d-orbitals empty for ${\mathrm{d}}^2{\mathrm{sp}}^3$ hybridization. NO₂ is a stronger ligand that promotes electron pairing and thus ${\mathrm{d}}^2{\mathrm{sp}}^3$hybridization.

So, option A is correct answer.