On adding an excess of $NH_3$ solution to $CuSO_4$ solution,the dark blue colour is due to the formation of which complex?

Assign $A, B, C, D$ from the given type of reaction: $Cd(CN)_2 \downarrow + 2KCN \longrightarrow K_2[Cd(CN)_4]$

The most stable complex among the following is

Ammonia reacts with copper ions in a basic medium to form the complex ion $[Cu(NH_3)_4]^{2+}$,but it does not form this complex ion in an acidic solution. Why?

Aluminium trifluoride is insoluble in anhydrous $HF$ but dissolves on addition of $NaF$. Aluminium trifluoride precipitates out of the resulting solution when gaseous $BF_3$ is bubbled through. Give reasons.

Amongst the following,the most stable complex is:
$(i)$ $[Fe(H_2O)_6]^{3+}$
$(ii)$ $[Fe(NH_3)_6]^{3+}$
$(iii)$ $[Fe(C_2O_4)_3]^{3-}$
$(iv)$ $[FeCl_6]^{3-}$