Of the following complex ions, the one that probably has the largest overall formation constant, $K_f$ is:

The probabity of forming the chelate is directly proportional to the formation constant, $K_f$_.

Since the probabity of forming the chelate is highest in the  ${\left[\mathrm{Co}(en{)}_3\right]}^{3+}$ the formation constant, $K_f$_.

Is increasing.

Hence option D is correct.