The pair having the same magnetic moment is: (Atomic numbers: Cr= 24, Mn= 25, Fe= 26, Co= 27)

The central atoms in the given complexes have the following electronic configurations:

The complexes ${\left[\mathrm{Cr}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}$ and ${\left[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}$ have the same number of unpaired electrons in the central atom, and, hence these will have the same magnetic moment.