Which one of the following species responds to an external magnetic field?

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

${\mathrm{Fe}}^{3+} : \left[\mathrm{Ar}\right]3{\mathrm{d}}^5$

Hybridisation : ${\mathrm{sp}}^3{\mathrm{d}}^2$

Magnetic nature : Paramagnetic (so this complex response to external magnetic field)

2. ${\left[\mathrm{Ni}{\left(\mathrm{CN}\right)}_4\right]}^{2-}$

${\mathrm{Ni}}^{3+} : \left[\mathrm{Ar}\right]3{\mathrm{d}}^8$

Hybridisation : ${\mathrm{dsp}}^2$

Magnetic nature : diamagnetic

3. ${\left[\mathrm{Co}{\left(\mathrm{CN}\right)}_6\right]}^{3-}$

${\mathrm{Co}}^{3+} : \left[\mathrm{Ar}\right]3{\mathrm{d}}^6$

Hybridisation : ${\mathrm{d}}^2{\mathrm{sp}}^3$

Magnetic nature : diamagnetic

4. $\left[\mathrm{Ni}{\left(\mathrm{CO}\right)}_4\right]$

$\mathrm{Ni} : \left[\mathrm{Ar}\right]3{\mathrm{d}}^8 4{\mathrm{s}}^2$

Hybridisation : ${\mathrm{sp}}^3$

Magnetic nature : diamagnetic