Explanation

(b)
Stannous chloride $(SnCl_{2})$ is a good reducing agent. It reduces $HgCl_{2}$ into Hg (grey precipitate), in two steps.
$SnCl_{2} + 2HgCl_{2} \rightarrow SnCl_{4} + Hg_{2}Cl_{2} \downarrow$
white
$SnCl_{2} + Hg_{2}Cl_{2} \rightarrow SnCl_{4} + 2Hg \downarrow$
grey