Answer:
(a) > (d) > (c) > (b)
Explanation:
Acidic character is based upon inductive effect of the functional group that are attached to parent chain.
1. Nitro is a -I group which makes phenol acidic due to increased resonance, 2. bromine is a -I group but it is weaker than nitro group
3. phenol is more acidic than cyclohexanol due to resonance stabilization of resonance and among halogen F>Cl>Br>I is the order of acidity
4. cyclohexanol will be weakest as it is not associated with any -I groups here.
so the order of acidity will be
(a). 4-nitropheno > (d). 4-bromophenol > (c). 2,6-difluorocyclohexanol > (b). cyclohexanol