In pure ethanol, molecules are strongly held together by intermolecular hydrogen bonds. When acetone is added, its molecules get in between ethanol molecules and break some of the hydrogen bonds between them. This weakens the intermolecular attractions ($A-B$ interactions are weaker than $A-A$ and $B-B$ interactions), increasing the escaping tendency of both molecules. Consequently, the mixture shows a positive deviation from Raoult's law and exhibits a higher vapour pressure.

Analysis of expressions:

• $P = P_1 + P_2$ is Dalton's law of partial pressures for a mixture of two volatile components.
• $P = P_1^\circ P_2^\circ$ is a product of pure vapour pressures, which has no physical meaning or basis in Dalton's or Raoult's laws.
• $P = P_1^\circ + (P_2^\circ - P_1^\circ)x_2$ is the standard Raoult's law total vapour pressure formula derived from $P = P_1^\circ(1-x_2) + P_2^\circ x_2$.
• $P_1 = P - P_2$ is a direct rearrangement of $P = P_1 + P_2$.

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 $\text{Na}_3\text{PO}_4$).