Q.Which compound is formed when excess of KCN is added to an aqueous solution of copper sulphate ?

With limited KCN, $Cu^{2+}$ gives unstable $Cu(CN)_2$ which decomposes to $CuCN$ + cyanogen gas; with excess KCN, the $CuCN$ precipitate redissolves as a soluble potassium cyanocuprate(I) complex.

The sequence is: $Cu^{2+} + 2CN^- \rightarrow Cu(CN)_2$ (unstable) $\rightarrow 2CuCN + (CN)_2\uparrow$ — cyanide reduces $Cu(II)$ to $Cu(I)$ while itself being oxidised to cyanogen. This $CuCN$ precipitate then dissolves in excess $KCN$ to give a soluble cyanocuprate(I) complex anion. Options (a) and (d) describe the intermediate/decomposition stage before excess cya …