The spin-only magnetic moment value of M^ n+- ion formed among Ni,...

$\mathrm{K}_2\left[\mathrm{NiCl}_4\right] \Rightarrow \mathrm{sp}^3$, Paramagnetic
\left[\mathrm{Zn}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right] \mathrm{Cl}_2 \Rightarrow \mathrm{sp}^3 \mathrm{~d}^2$, Diamagnetic<br />$\mathrm{K}_3\left[\mathrm{Mn}(\mathrm{CN})_6\right] \Rightarrow \mathrm{d}^2 \mathrm{sp}^3, Paramagnetic<br />\left[\mathrm{Cu}\left(\mathrm{PPh}_3\right)_3\mathrm{I}\right] \Rightarrow \mathrm{sp}^3, Diamagnetic
Hence the value of $n$ is 2
Least value of enthalpy of atomisation among Ni ,
$\mathrm{Zn}, \mathrm{Mn}$ and Cu is of Zn
$$\begin{aligned}<br />& \mathrm{Zn}^{+2}:-[\mathrm{Ar}] 3 \mathrm{~d}^{10} \\<br>& \mu=0<br />\end{aligned}$$