(a) It is the magnitude difference in energy between the two sets of d-orbital i,e ${\mathit{t}}_{2\mathit{g}}$ and eg. electronic configuration of ${\mathit{d}}^5$ if ${\mathrm{\Delta }}_0>\mathit{P}$ is ${\mathit{t}}_{2\mathit{g}}^5$ eg Because in a strong field ligand pairing of electrons takes place for eg ${\left[\mathit{N}\mathit{i}{\left(\mathit{C}\mathit{N}\right)}_4\right]}^{2-}$
$-\mathit{C}\mathit{N}$ is a strong field ligand(b) $\mathit{N}\mathit{i}=28,3{\mathit{d}}^{8}4{\mathit{s}}^2$
${\mathit{N}\mathit{i}}^{2+}=3{\mathit{d}}^{8}4{\mathit{s}}^0$
$\left[\mathit{N}\mathit{i}{\left(\mathit{C}\mathit{N}\right)}_4\right]$ is a strong field ligand.$={\mathit{d}\mathit{s}\mathit{p}}^2$ hybridization showing square planar geometry
$=$ All electrons are paired so it is diamagnetic in nature.
$\left[\mathit{N}\mathit{i}{\left(\mathit{C}\mathit{O}\right)}_4\right]$
$\mathit{N}\mathit{i}=28,3{\mathit{d}}^{8}4{\mathit{s}}^2$
$\mathit{N}\mathit{i}=$ valency is zero
$={\mathit{s}\mathit{p}}^3$ hybridization
$\to$ showing tetrahedral geometry
$\to$ All electrons are paired
so it is diamagnetic in nature.