Which of the following complexes will exhibit maximum attraction to...

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

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

So Option D is correct.