Answer: $(1) \rightarrow d\; \quad (2) \rightarrow c \; \quad (3) \rightarrow b\; \quad (4) \rightarrow a$

The elements are in Period 4 so the general configuration is $ns^2(n-1)d^x$

1. Cr is the 4th element in the series but there is an exception made for Cr because the listed electronic configurations are energetically favoured $\rightarrow 3d^54s^1$. To form $Cr^{2+}$, it must lose one 4s and one 3d $\rightarrow [Ar] 3d^4$

2. Zn is the last element in the series and to form $Zn^{2+}$ it must lose two 4s electrons $\rightarrow [Ar] 3d^{10}$

2. Ni is the 8th element in the series and to form $Ni^{3+}$ it must lose two 4s electrons and one 3d $\rightarrow [Ar] 3d^{7}$

4. Cu is the 9th element in the series but there is an exception made for Cu because the listed electronic configurations are energetically favoured $\rightarrow 3d^{10}4s^1$. To form $Cu^{2+}$, it must lose one 4s and one 3d $\rightarrow [Ar] 3d^9$