Which of the following has the maximum number of unpaired ' \(d\) ' electrons?

\(
\begin{aligned}
\mathrm{Zn}(30)=& {[\mathrm{Ar}] 3 \mathrm{~d}^{10}, 4 \mathrm{~s}^{2} } \\
\mathrm{Zn}^{2+} &=[\mathrm{Ar}] 3 \mathrm{~d}^{10} \text { (no unpaired electron) } \\
\mathrm{Fe}(26) &=[\mathrm{Ar}] 3 \mathrm{~d}^{6}, 4 \mathrm{~s}^{2} \\
\mathrm{Fe}^{2+} &=[\mathrm{Ar}] 3 \mathrm{~d}^{6} \text { (four unpaired electrons) }
\end{aligned}
\)



\(\begin{aligned} \mathrm{Ni}(28) &=[\mathrm{Ar}] 3 \mathrm{~d}^{8}, 4 \mathrm{~s}^{2} \\ \mathrm{Ni}^{3+} &=[\mathrm{Ar}] 3 \mathrm{~d}^{7} \text { (three unpaired electrons) } \end{aligned}\)


\(
\begin{aligned}
\mathrm{Cu}(29) &=[\mathrm{Ar}] 3 \mathrm{~d}^{10}, 4 \mathrm{~s}^{1} \\
\mathrm{Cu}^{+} &=[\mathrm{Ar}] 3 \mathrm{~d}^{10} \text { (no unpaired electron) }
\end{aligned}
\)