According to the Hardy-Schulze rule:

• $\text{AgI}/\text{Ag}^+$ is a positively charged sol due to preferential adsorption of $\text{Ag}^+$ ions.
• Coagulation of a positively charged sol is carried out by the anions of the added electrolyte.
• The greater the valency of the coagulating anion, the greater is its coagulating (flocculating) power:
  $$\text{PO}_4^{3-} (|-3|) \gt \text{S}^{2-} / \text{SO}_4^{2-} (|-2|) \gt \text{Cl}^- (|-1|)$$
• Because $\text{PO}_4^{3-}$ carries the highest negative charge, $\text{Na}_3\text{PO}_4$ has the highest coagulating power, which the NTA official key accepted as option 2. (Note: In strict textbook definitions, coagulating value is inversely proportional to coagulating power, where $\text{NaCl}$ would have the largest numerical millimolar concentration, but standard examination keys consistently equate highest coagulating effectiveness with $\text{Na}_3\text{PO}_4$).