When salt solution ' A ' is treated with dilute $\mathrm{HCl}$ it gives off gas ' B ', which when passed through a filter paper, moistened with potassium iodate and starch solution, gives deep blue coloration. ' A ' and ' B ' respectively are :

(a) $$\begin{gathered} \underset{\mathrm{sodium} \mathrm{sulphide}}{{\mathrm{Na}}_2{\mathrm{SO}}_3}\left(\mathrm{aq}\right)+\underset{\mathrm{di} \mathrm{hydrochloric} \mathrm{acid} }{2\mathrm{HCl}(}\mathrm{aq})\to \\ \underset{\mathrm{sodium} \mathrm{chloride} }{2\mathrm{NaCl}}(\mathrm{aq})+\underset{\mathrm{water} }{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}+\underset{\mathrm{sulphur} \mathrm{dioxide}}{{\mathrm{SO}}_2(}\mathrm{g}) \end{gathered}$$

Therefore, the reaction between dil. HCl and Sodium sulfite $\left({\mathrm{Na}}_2{\mathrm{SO}}_3\right)$ produces Sulphur dioxide gas$\left({\mathrm{SO}}_2\right)$, Sodium chloride$\left(\mathrm{NaCl}\right)$ and water.

(c) ${\mathrm{Na}}_2{\mathrm{S}}_{\left(\mathrm{aq}\right)}+2{\mathrm{HCl}}_{\left(\mathrm{aq}\right)}\to 2{\mathrm{NaCl}}_{\left(\mathrm{aq}\right)}+{\mathrm{H}}_2{\mathrm{S}}_{\left(\mathrm{g}\right)}$

(d)$\underset{sodium bisulphite}{{\mathrm{NaHSO}}_3}+\underset{\mathrm{dil}.}{\mathrm{HCl}}⟶\underset{\mathrm{sodium} \mathrm{chloride} }{\mathrm{NaCl}}+\underset{\mathrm{water} }{{\mathrm{H}}_2\mathrm{O}}+\underset{\mathrm{sulphur} \mathrm{dioxide}}{{\mathrm{SO}}_2^{\uparrow }}$

Hence, the correct option is a, c, d.