HCl was passed through a solution of CaCl _ 2 , MgCl _ 2 and NaCl ....

When $HCl$ is passed thorugh the mixture ${Cl}^{-}$ ion concentration increases. Hence ionic product \left[{Na}^{+}\right]\left[{Cl}^{-}\right]
becomes more than solubility product. So $NaCl$ will precipitate out.
Filtrate $\overset{HClgaspassed}{\to}$ pure $NaCl$ precipitation (common ion effect)