The acidity of phenols is determined by the stability of the phenoxide ion formed after losing a proton.

1. Electron-Withdrawing Groups (EWG): Groups like $-NO_2$ withdraw electrons, dispersing the negative charge on the oxygen and stabilizing the phenoxide ion, thus increasing acidity.

• para-nitrophenol (d): The $-NO_2$ group exerts both a strong electron-withdrawing resonance effect ($-R$) and an inductive effect ($-I$) at the para position. This makes it the most acidic.
• meta-nitrophenol (c): The $-NO_2$ group at the meta position exerts only the electron-withdrawing inductive effect ($-I$). The resonance effect does not operate at the meta position. Therefore, it is less acidic than the para isomer but more acidic than phenol.

2. Standard: Phenol (a) has no substituents and serves as the reference.
3. Electron-Donating Groups (EDG): Groups like methyl ($-CH_3$) release electrons (via $+I$ effect and hyperconjugation), intensifying the negative charge on the oxygen and destabilizing the phenoxide ion, thus decreasing acidity.

• Methyl phenol (b): Due to the electron-donating nature of the methyl group, it is less acidic than phenol.

Conclusion: The order of acidity is d > c > a > b.