The substitution reaction of

with OH^- to give

is

Correct option is C

Replacement of a ligand from coordination sphere by OH^- is called base hydrolysis. The most suitable mechanism has been given by Garrick which is called as S_N1cB (substitution, nucleophilic, first order, conjugate base) mechanism.
The mechanism of Co (III) complexes containing amine ligands is shown below:

The first step involves the removal of proton from NH₃ group by OH^- in a rapid acid-base equilibrium forming amide complex ion of lower charge (a conjugate base of original amine complex). Then this conjugate base dissociates to loss Cl^- in slow step [reaction (ii)] and finally it forms the hydroxo complex rapidly [reaction (iii)]. Thus, the second step is the rate determining step.
In amido complex (i.e., conjugate base) the NH₂^- ligand has the tendency to form π-bond by donating its lone pair of electrons to the metal cation, Co (III).

The formation of Co-N π-bond increases the electron density on cobalt which in turn repels the labile ligand, Cl^- and the π-bond also stabilizes the five-coordinate intermediate. The donor π-bonding is strong evidence of an S_N1 mechanism. Since the dissociation step which is the rate determining step uses the conjugate base of the original complex, therefore, it is called as S_N1cB mechanism. It has been suggested that, the hydroxide ion abstract proton from the NH₃ group which is trans to the leaving group, i.e., the halo group.
The rate determining step is dissociation of conjugate base [reaction (ii)] but since its concentration depends on the OH^- ion concentration through equilibrium and the reaction rate is proportional to OH^- ion concentration.
For base hydrolysis it is essential that there should be at least one ligand in a complex which have acidic protons.
The rate of hydrolysis of [Co (CN)₅Cl]^3- is independent of OH^- concentration because of the absence of acidic proton.