Which of the following has a lower stretching frequency for $\mathrm{C}-\mathrm{O}$a bond?

${\left[\mathrm{Mn}(\mathrm{CO}{)}_6\right]}^{+}$ has lower stretching frequency for  $\mathrm{C}-\mathrm{O}$ bond. 

The metal has a positive charge, which favors the process of back donating.

The electrons from the metal fill the π− antibonding orbital of CO, they weaken the carbon-oxygen bond.

Hence, option A is correct.