Aqueous solution of a gas $\text{'}\mathrm{X}\text{'}$decolourises an acidified ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$ solution and on passing ${\mathrm{H}}_2\mathrm{S}$ in the solution gives white turbidity  $\text{'}\mathrm{Y}\text{'}$. Here $\text{'}\mathrm{X}\text{'}$and $\text{'}\mathrm{Y}\text{'}$ respectively are :

Aqueous solution of Sulfur dioxide ${\mathrm{SO}}_2$ decolourises an acidified ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$ solution and on passing hydrogen sulfide in the solution gives white turbidity of  ${\mathrm{PbSO}}_4$(lead sulphate)

Acidified potassium dichromate solution is decolored by sulfur dioxide. Oxidizing sulphur dioxide produces sulfuric acid. Chromic sulphate is formed by reducing potassium dichromate. The solution orange colour changes to green.

$3{\mathrm{SO}}_2+{\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7+{\mathrm{H}}_2{\mathrm{SO}}_4\to {\mathrm{K}}_2{\mathrm{SO}}_4+{\mathrm{Cr}}_2{\left({\mathrm{SO}}_4\right)}_3+{\mathrm{H}}_2\mathrm{O}$

Sulfuric acid is produced by heating sulphur dioxide and hydrogen peroxide together, then cooling the mixture.

${\mathrm{SO}}_2+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{H}}_2{\mathrm{SO}}_4$

A white precipitate of barium sulphate that is insoluble in dilute HCl is produced when barium chloride is introduced to sulfuric acid.

${\mathrm{H}}_2{\mathrm{SO}}_4+{\mathrm{BaCl}}_2\to {\mathrm{BaSO}}_4+2\mathrm{HCl}$

White turbidity is produced after passing ${\mathrm{H}}_2\mathrm{S}$ through the solution because of the production of Sulphur.

$2{\mathrm{H}}_2\mathrm{S}+{\mathrm{SO}}_2\to 2{\mathrm{H}}_2\mathrm{O}+3\mathrm{S}\downarrow$

Hence correct option is A