Explanation

(b)
When an electron attracting group (like $–NO_{2}, - CI$) is attached to the phenol ring, it stabilises the negative charge on the oxygen of phenoxide ion. Due to this reason acidic character of phenol increases. But when an electron donating group (like $–CH_{3}$) is attached to the phenol ring, it destabilises the ring and hence, acidic character of phenol decreases. Thus, the correct order of acidic character is
p- nitrophenol > p-chlorophenol > phenol > o-cresol.