The complex ions ${\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]}^{3-}$ and ${\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]}^{4-}$ :

Since ${\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]}^{4-}$ and ${\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]}^{3-}$has zero or one unpaired electrons. As a result, they both differ in their magnetic property.
Electronic configuration of ${\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]}^{4-}$  is $4s^{2}3d^6$( diamagnetic).
Electronic configuration of ${\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]}^{3-}$  is $4s^{2}3d^5$ (paramagnetic).

Thus, the correct option is c.