The strongest acid from the following is

An acid is stronger if it can easily lose a hydrogen ion (H+) and form a stable negative ion (phenoxide ion).

• p-nitrophenol: Contains a nitro group (-NO2). The -NO2 group is a Strong EWG due to both its powerful resonance (-M) and inductive (-I) effects. It strongly pulls electron density away from the ring, making it the most stable phenoxide ion.
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• p-chlorophenol: Contains a chlorine atom (-Cl). Chlorine is a Weak EWG. While its inductive effect (-I) withdraws electrons, its resonance effect (+M) donates some electrons back, making it significantly less acidic than the nitro-substituted version.
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• Phenol: Has no substituents on the benzene ring, representing the standard baseline reference point.
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• p-methylphenol: Contains a methyl group (-CH3). The methyl group is an EDG due to hyperconjugation and inductive effect (+I). It destabilizes the negative charge, making this molecule the least acidic of all.

p-nitrophenol > p-chlorophenol > Phenol > p-methylphenol