The acidic strength of phenols is governed by the stability of the phenoxide ion formed after the loss of a proton.

1. Electron-Withdrawing Groups (EWG): Substituents like $-NO_2$, $-CN$, and halogens withdraw electrons from the benzene ring (via $-I$ and $-R$ effects). This delocalises the negative charge on the phenoxide oxygen, stabilizing the ion and increasing acidity , .
2. Electron-Donating Groups (EDG): Substituents like alkyl groups (e.g., $-CH_3$) or alkoxy groups release electrons (via $+I$ or $+R$ effects). This intensifies the negative charge on the phenoxide oxygen, destabilizing the ion and decreasing acidity.

Given the correct order III > I > II, it can be inferred that:

• Compound (III) likely contains an electron-withdrawing group (making it the most acidic).
• Compound (II) likely contains an electron-donating group (making it the least acidic).
• Compound (I) is likely the unsubstituted phenol or has an intermediate substituent effect.