Explanation

(b)
Phenols are much more acidic than alcohol due to the stabilisation of phenoxide ion resonance.
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Phenoxide ion is stabilised due to following resonating structures.
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While, in alcohols
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$ortho$ nitrophenol is most acidic because in $–NO_{2}$ electron attracting group is attached to $ortho$ position which helps in stabilising the negative charge on the oxygen of phenoxide ion. Hence, due to this reason acidic character of phenol is increased, while on attachment of $–CH_{3}$ group (electron donating group) acidic strength of phenol is decreased in cresol due to the destabilisation of phenoxide ion.