a
According to \(MOT\) (If \(z\) is internuclear axis)

The configuration of

\(\mathrm{CN}^{-}: \sigma_{1 \mathrm{s}}^{2}, \sigma_{1 \mathrm{s}}^{* 2}, \sigma_{2 \mathrm{s}}^{2}, \sigma_{2 \mathrm{s}}^{*2}, \pi_{2 \mathrm{p}_{\mathrm{x}}}^{2}=\pi_{2 \mathrm{p}_{\mathrm{y}}}^{2}, \sigma_{2 \mathrm{p}_{\mathrm{z}}}^{2}\)

Bond order \(=\frac{1}{2}(10-4)\)

\(=3\)

\(\mathrm{CN}^{-}\) is diamagnetic due to absence of unpaired electron