Phenol is less acidic than . . . .
(b) o - nitrophenol
(d) o - methoxyphenol
(b) In o-nitrophenol, nitro group is present at ortho position. Presence of electron withdrawing group at ortho position increases the acidic strength. On the other hand, in o-methylphenol and in o-methoxyphenol, electron releasing group (- CH3, - OCH3) are present.
Presence of these groups at ortho or para positions of phenol decreases the acidic strength of phenols. So, phenol is less acidic than o - nitrophenol.