${\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3\left(aq\right)+\text{ Reagent }\left[A\right]⟶\underset{\underset{[\mathrm{Black} \mathrm{ppt}. \mathrm{of} \mathrm{metal} \mathrm{sulphide}] }{\mathrm{on} \mathrm{warming} \swarrow } \underset{\mathrm{Soluble} \mathrm{complex} \mathrm{of}\left[\mathrm{B}\right] }{\searrow + \mathrm{Excess} {\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3}}{\mathrm{white} \mathrm{ppt}. \mathrm{of} \mathrm{B}}$

Among the following, how many are possible 'A'?

$\begin{array}{l}{\mathrm{AgNO}}_3,{\mathrm{BaCl}}_2,\mathrm{Pb}(\mathrm{OAc}{)}_2,{\mathrm{HgCl}}_2,{\mathrm{CaCl}}_2,{\mathrm{CuSO}}_4\text{, }\\ {\mathrm{FeCl}}_3.\end{array}$

Regents ${\mathrm{AgNO}}_3,\mathrm{Pb}(\mathrm{OAc}{)}_2,{\mathrm{HgCl}}_2,{\mathrm{CuSO}}_4$ gives white ppt when reacting with ${\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3$.

${\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3\left(aq\right)+\text{ Reagent }\left[A\right]⟶\underset{\underset{[\mathrm{Black} \mathrm{ppt}. \mathrm{of} \mathrm{metal} \mathrm{sulphide}] }{\mathrm{on} \mathrm{warming} \swarrow } \underset{\mathrm{Soluble} \mathrm{complex} \mathrm{of}\left[\mathrm{B}\right] }{\searrow + \mathrm{Excess} {\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3}}{\mathrm{white} \mathrm{ppt}. \mathrm{of} \mathrm{B}}$

Thus, the correct answer is 4.