The salt used for performing bead test in inorganic qualitative analysis is

Question

$\begin{array}{1 1}(a)\;Na(NH_4)HPO_4.4H_2O\\(b)\;K_2SO_4Al_2(SO_4)_3.24H_2O\\(c)\;FeSO_4(NH_4)_2SO_4.6H_2O\\(d)\;CuSO_4.2H_2O\end{array}$