Which of the following ionic substances will be most effective in precipitating the sulphur sol?

Higher is the charge on cation, higher is the precipitating power. Fe2(SO4)3 will be most effective in precipitating the sulphur sol. Sulphur sol is negatively charged colloid. It will be precipitated by cations. Among the given ionic substances, Fe3+ ion has maximum charge and hence maximum precipitating power. Hence, option C is correct.