The correct option is B Na3PO4
According to Hardy Schulze law, as the valency of flocculating ions increases, the coagulating power of the ions also increases.
The coagulation power depends on the charge of the ions. As the charge increases, the coagulation power also increases.
In the given options Na3PO4 has the PO43− ion which has the maximum charge, hence it will have the maximum coagulating value for AgIAg+.