Of the species $\mathrm{NO},{\mathrm{NO}}^{+},{\mathrm{NO}}^{2+}$ and NO^-, the one with minimum bond strength is

$\mathrm{BO}=\frac{{\mathrm{N}}_{\mathrm{b}}-{\mathrm{N}}_{\mathrm{a}}}{2}$

Energy order of MO s

${\mathrm{\sigma }}_{1\mathrm{s}}{\mathrm{\sigma }}_{1\mathrm{s}}^{\ast }{\mathrm{\sigma }}_{2\mathrm{s}}{\mathrm{\sigma }}_{2\mathrm{s}}^{\ast }{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{x}}}={\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{y}}}{\mathrm{\sigma }}_{2{\mathrm{p}}_{\mathrm{z}}}{\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{x}}}^{\ast }={\mathrm{\pi }}_{2{\mathrm{p}}_{\mathrm{y}}}^{\ast }$

$\text{ We know, Bond length }\propto \frac{1}{\mathrm{BO}\text{ (Bond order) }}\propto \frac{1}{\text{ Bond strength }}$

So, the order of bond strength : ${\mathrm{NO}}^{+}>\mathrm{NO}={\mathrm{NO}}^{2+}>{\mathrm{NO}}^{-}$

So, NO^- has minimum bond strength.