$$\begin{gathered} \stackrel{\left(\mathrm{C}\right)}{\mathrm{Yellow} \mathrm{ppt} \stackrel{{\mathrm{Hg}}_2{\left({\mathrm{NO}}_3\right)}_2}{\leftarrow } }\stackrel{\left(A\right) }{\left(Salt\right)} \stackrel{\text{ dil. }{\mathrm{H}}_2{\mathrm{SO}}_4}{\leftarrow } \stackrel{\left(B\right)}{Odourless gas } \\ \downarrow {\mathrm{CaCl}}_2\text{ sol. } \\ \stackrel{\left(D\right)}{White ppt.} \end{gathered}$$

In the above scheme, suggest the most probable formula of A.

When sodium carbonate is passes through the sodium carbonate following reaction occur,

${\mathrm{CaCl}}_2\left(\mathrm{aq}\right)+{\mathrm{Na}}_2{\mathrm{CO}}_3\left(\mathrm{aq}\right)\to {\mathrm{CaCO}}_3\left(\mathrm{s}\right)+2\mathrm{NaCl}\left(\mathrm{aq}\right)$

When mercuric chloride is heated with sodium carbonate solution, mercuric oxide precipitates out as a yellow substance.

${\mathrm{HgCl}}_2+{\mathrm{Na}}_2{\mathrm{CO}}_3\to \mathrm{HgO}+2\mathrm{NaCl}+{\mathrm{CO}}_2$

Hence correct option is A.