Among the following metal carbonyl carbonyls, the $\mathrm{C}-\mathrm{O}$ bond is strongest in :

${\left[\mathrm{Mn}(\mathrm{CO}{)}_6\right]}^{+}$ because it exhibits the strongest $\mathrm{C}=\mathrm{O}$ bond and the weakest m-c bond, as well as the least Mlectron density and sigmoidal electron density.

As the charge on $\text{ C.A. }\downarrow ,C-\mathrm{O}$ bond strength $\uparrow$

or $\mathrm{C}-\mathrm{O}$ bond order,  $\uparrow$ or $\mathrm{C}-\mathrm{O}$ bond length$\downarrow$

Hence correct option A.