The order of strength of carboxylic acid is Find the correct order.

The acidic strength of a carboxylic acid depends on the stability of its conjugate base (carboxylate anion). Electron-withdrawing groups ($-I$ effect) stabilize the anion and increase acidic strength.

In the given compounds, the oxygen atom in the ring is more electronegative than carbon and exerts a $-I$ effect.

In structure (I), the oxygen atom is at the 4-position relative to the carbon bearing the $-\text{COOH}$ group.

In structure (II), there is no electronegative heteroatom in the ring, making it the least acidic among the three.

In structure (III), the oxygen atom is at the 3-position relative to the $-\text{COOH}$ group.

The $-I$ effect is distance-dependent; its strength decreases as the distance between the electron-withdrawing group and the acidic group increases. Since the oxygen atom in (III) is closer to the $-\text{COOH}$ group than in (I), the $-I$ effect is stronger in (III) compared to (I).

Therefore, the carboxylate anion of (III) is more stable than that of (I), making (III) a stronger acid than (I). Both (I) and (III) are stronger acids than (II) due to the presence of the electron-withdrawing oxygen atom.

The correct order of acidic strength is $\text{III} > \text{I} > \text{II}$.

Answer: $\text{III} > \text{I} > \text{II}$