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

The ion which is responsible for neutralization of charge on the colloidal particles is called the coagulating ion or Flocculating ion. So, according to this rule, greater the charge on an oppositely charged ion, greater is its coagulation power.

To coagulate negative sol, cation with higher charge has higher coagulation value.

So answer is $\mathrm{Ba}+2$