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The correct acidic order of the following is : -
A
I > II > III
B
III > I > II
C
II > III > I
D
I > III > II
Detailed Solution
Phenol is acidic because the phenoxide ion formed after losing $H^+$ is stabilised by resonance.
An electron withdrawing group such as $-NO_2$ (by $-I$ and $-R$ effects) further stabilises the phenoxide ion by spreading its negative charge, so p-nitrophenol ($pK_a \approx 7.1$) is more acidic than phenol ($pK_a \approx 10$).
An electron donating group such as $-CH_3$ (+I effect and hyperconjugation) destabilises the phenoxide ion, so p-cresol ($pK_a \approx 10.2$) is less acidic than phenol.
So the order is III $\gt$ I $\gt$ II.
An electron withdrawing group such as $-NO_2$ (by $-I$ and $-R$ effects) further stabilises the phenoxide ion by spreading its negative charge, so p-nitrophenol ($pK_a \approx 7.1$) is more acidic than phenol ($pK_a \approx 10$).
An electron donating group such as $-CH_3$ (+I effect and hyperconjugation) destabilises the phenoxide ion, so p-cresol ($pK_a \approx 10.2$) is less acidic than phenol.
So the order is III $\gt$ I $\gt$ II.