The correct option is D CN−
Cu2+ ions will react with CN− and SCN− forming [Cu(CN)4]3− and [Cu(SCN)4]3− leading the reaction in the backward direction.
Cu2++2CN−→Cu(CN)2
2Cu(CN)2→2CuCN+(CN)2
CuCN+3CN−→[Cu(CN)1]3−
Cu2++4SCN−→[Cu(SCN)4]3−
Cu2+ also combines with CuCl2 which reacts with Cu to produce CuCl pushing the reaction in the back ward direction.
CuCl2+Cu→2CuCl↓