Which of the following complexes will exhibit maximum attraction to an applied magnetic field?

Complex having maximum number of unpaired electrons will exhibit maximum attraction to applied magnetic field.

${\left[\mathrm{Zn}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+} \to {\mathrm{d}}^{10} \mathrm{system}, {{\mathrm{t}}^6}_{2\mathrm{g}} {\mathrm{eg}}^4, 0$ unpaired electrons
${\left[\mathrm{Co}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}\to {\mathrm{d}}^7 \mathrm{system}, {{\mathrm{t}}^5}_{2\mathrm{g}} {\mathrm{eg}}^2, 3$ unpaired electrons
${\left[\mathrm{Co}(\mathrm{en}{)}_3\right]}^{3+}\to {\mathrm{d}}^6 \mathrm{system}, {{\mathrm{t}}^6}_{2\mathrm{g}} {\mathrm{eg}}^0, 0$ unpaired electrons
${\left[\mathrm{Ni}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}\to {\mathrm{d}}^8 \mathrm{system}, {{\mathrm{t}}^6}_{2\mathrm{g}} {\mathrm{eg}}^2, 2$ unpaired electrons.

So Option D is correct.