Complex having maximum number of unpaired electrons will exhibit maximum attraction to applied magnetic field.
[Zn(H2O)6]2+→d10system,t62geg4,0 unpaired electrons
[Co(H2O)6]2+→d7system,t52geg2,3 unpaired electrons
[Co(en)3]3+→d6system,t62geg0,0 unpaired electrons
[Ni(H2O)6]2+→d8system,t62geg2,2 unpaired electrons.
So Option D is correct.