Which of the following is a diamagnetic complex

Among the given option (a) is a diamagnetic complex.

(a) $\left[Co\left({NH}_{3}\right)6\right]3 + ⇒ Co\left(27\right) ⇒ {Co}^{3 +} = 3d^{6}4s^{0}$.

Thus, has no unpaired electrons and is a diamagnetic complex.

(b) \left[{NiCl}_{4}\right]2 - ⇒ Ni\left(28\right) ⇒ {Ni}^{2 +} = 3d^{8}4s^{0}$~</p><p>Thus, has two unpaired electrons is paramagnetic.</p><p>(c) \left[{CuCl}_{4}\right]2 - ⇒ Cu\left(29\right) ⇒ {Cu}^{2 +} = 3d^{9}4s^{0}$

Thus, has one unpaired electrons and is paramagnetic.

(d) \left[Fe\left({H}_{2}O\right)6\right]3 + ⇒ Fe\left(26\right) ⇒ {Fe}^{3 +} = 3d^{5}4s^{0}

Thus, has five unpaired electrons and is paramagnetic.