$CuSO_4$ when reacts with KCN forms CuCN, which is insoluble in water. It is soluble in excess of KCN, due to formation of the following complex : -

Question

( A ) $CuCN_2$
( B ) $K_3[Cu(CN)_4]$
( C ) $K_2[Cu(CN)_4]$
( D ) $Cu[KCu(CN)_4]$