$\begin{array} {ll}
\text{Element} & \text{Electron Configuration}\\
(1)\; Cr^{2+} & a.\;[Ar]3d^9 \\
(2) \; Zn^{2+} & b.\;[Ar]3d^7 \\
(3) \;Ni^{3+} & c.\; [Ar]3d^{10}\\
(4) \; Cu^{2+} & d.\;[Ar]3d^4
\end{array}$
$\begin {array} {1 1} (1) \rightarrow b\; \quad (2) \rightarrow c \; \quad (3) \rightarrow d\; \quad (4) \rightarrow c \\ (1) \rightarrow d\; \quad (2) \rightarrow c \; \quad (3) \rightarrow b\; \quad (4) \rightarrow a \\ (1) \rightarrow d\; \quad (2) \rightarrow b \; \quad (3) \rightarrow a\; \quad (4) \rightarrow c \\ (1) \rightarrow c\; \quad (2) \rightarrow a \; \quad (3) \rightarrow d\; \quad (4) \rightarrow b\end {array}$
