Which of the following is diamagnetic ?

 

 

$[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6{]}^{3-}$ is more diamagnetic.

${\mathrm{CN}}^{-}$=  Strong field ligand, so in d orbital, it will pair up the electrons.

The crystal structure of $[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6{]}^{3-}$ is 

$[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6{]}^{3-}{\mathrm{H}}_2\mathrm{O}$. 

The ${\mathrm{CN}}^{-}$ ligand creates a strong field (since it's a strong field ligand), and the d orbital of the Fe atom will pair up with its partner because of this.