$\begin{array}{l}{\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3\stackrel{\text{ dil. }{\mathrm{H}}_2{\mathrm{SO}}_4}{\to }{\mathrm{Na}}_2{\mathrm{SO}}_4+\mathrm{S}(\downarrow )+\text{'}\mathrm{B}{\text{'}}^{ }\text{ (gas) }\\ { }^{\text{'}}{\mathrm{B}}^{\mathrm{\prime }}+\mathrm{Ba}(\mathrm{OH}{)}_2{⟶}^{ }\text{'}\mathrm{C}\text{'}(\downarrow )+{\mathrm{H}}_2\mathrm{O}\\ \text{ (white ppt.) }\\ \downarrow {\mathrm{B}}^{\mathrm{\prime }}\\ \text{ Clear solution ' }{\mathrm{D}}^{\mathrm{\prime }}\end{array}$

Here '${\mathrm{H}}_2\mathrm{S}$' and '$\mathrm{D}$' are :

The following reactions are obtained,

${\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3\stackrel{\text{ dil. }{\mathrm{H}}_2{\mathrm{SO}}_4}{\to }{\mathrm{Na}}_2{\mathrm{SO}}_4+\mathrm{S}(\downarrow )+{\mathrm{SO}}_2$

${\mathrm{SO}}_2+\mathrm{Ba}(\mathrm{OH}{)}_2\to \mathrm{Ba}{\left({\mathrm{HSO}}_3\right)}_2(\downarrow )+{\mathrm{H}}_2\mathrm{O}$

When sodium thiosulfate reacts with acid, Sulfur dioxide is obtained which is a harmful byproduct of the process.

Sodium bisulfate is produced when sodium sulphate and sulfuric acid react

${\mathrm{Na}}_2{\mathrm{SO}}_4+{\mathrm{H}}_2{\mathrm{SO}}_4\rightleftharpoons 2\mathrm{NaHSO}$

Hence correct option is D