For the coagulation of a negative sol, the species below, that has the highest flocculating power is:

According to the Hardy-Schulze rule, the coagulating power of an ion increases with its charge (valency). For coagulation of a negative sol, we need cations (positively charged ions), since opposite charges cause coagulation.

Among the given options, $$\text{SO}_4^{2-}$$ and $$\text{PO}_4^{3-}$$ are anions, so they cannot coagulate a negative sol. The cations are $$\text{Ba}^{2+}$$ and $$\text{Na}^+$$.

Since $$\text{Ba}^{2+}$$ carries a higher positive charge (+2) than $$\text{Na}^+$$ (+1), it has greater flocculating power for a negatively charged sol. Therefore, $$\text{Ba}^{2+}$$ has the highest flocculating power among the given species.