Which one of the following compounds has the most acidic nature?

Key Idea Presence of electron withdrawing substituent increases the acidity while electron releasing substituent, when present, decreases the acidity. Phenyl is an electron withdrawing substituent while $-\mathrm{CH}_3$ is an electron releasing substituent. Moreover, phenoxide ion is more resonance stabilised as compared to benzyloxide ion, thus releases proton more easily. That's why is a strong acid among the given.