Which of the following compounds is diamagnetic?

Which complex among the following is most paramagnetic?

Compounds with spin-only magnetic moment equivalent to five unpaired electrons are

Which of the following complex ions is expected to absorb light in the visible region?

$A$ coordination complex of type $MX_2Y_2$ ($M$ = metal ion; $X, Y$ = monodentate ligands) can have either a tetrahedral or a square planar geometry. The maximum number of possible isomers in these two cases are respectively:

$A$ solution of $[Ni(H_{2}O)_{6}]^{2+}$ is green but a solution of $[Ni(CN)_{4}]^{2-}$ is colourless. Explain.