In which of the following metal carbonyls, the $\mathrm{C}-\mathrm{O}$ bond length is minimum?

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

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

In ${\left[\mathrm{Mn}(\mathrm{CO}{)}_6\right]}^{+}$ $\mathrm{C}-\mathrm{O}$ bond length is minimum as the bond order is maximum in this molecule  among all.

Hence correct option B.