$\begin{array}{r}{\mathrm{FeCl}}_3+\underset{\text{ (salt solution) }}{\text{'}\mathrm{X}\text{'}}⟶\underset{(\mathrm{dark}violet \mathrm{colouration})}{\text{'}\mathrm{Y}\text{'}}\\ \downarrow \text{ on standing }\\ \text{ [Colour disappears] }\end{array}$

Here 'X' and 'Y' are :

When ferric chloride is added to a sodium thiosulphate solution, ferric thiosulphate forms, giving the mixture its violet colour.

$2{\mathrm{FeCl}}_3+3^{\mathrm{\prime }}{\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3^{\mathrm{\prime }}\left(\text{ salt solution }\right){⟶}^{\mathrm{\prime }}{\left[\mathrm{Fe}{\left({\mathrm{S}}_2{\mathrm{O}}_3\right)}_2\right]}^{-}+\mathrm{NaCl}$

Correct option is B