(C) The correct answer is $(C)$.In $CuCN$,the copper ion is $Cu^+$. The electronic configuration of $Cu^+$ is $[Ar] 3d^{10} 4s^0$.Since all electrons

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(C) The correct answer is $(C)$.In $CuCN$,the copper ion is $Cu^+$. The electronic configuration of $Cu^+$ is $[Ar] 3d^{10} 4s^0$.Since all electrons in the $3d$ subshell are paired,there are no unpaired electrons available for $d-d$ transitions.Therefore,$CuCN$ is expected to be colourless.