An arsenious sulphide sol carries a negative charge. The maximum precipitating power of this sol is possessed by:

Arsenious sulphide sol has a negative charge, therefore positive ions can cause it to coagulate.
The ability of the flocculating ion to produce precipitation increases with valency.
${\mathrm{AlCl}}_3$  has a positive cation with the greatest valency of $+3$ in each of the compounds listed. It has the most precipitating power as a result.