Question

Which is most acidic and why?
A.
B.
C.
D.

Answer 1

Use the concept of inductive effect and mesomeric effect. Compare the inductive effect of the $N{O_2}$ group at different positions. Hydrogen bonding also plays an important role while comparing the acidity of compounds. It can be intramolecular hydrogen bonding.

Complete answer: Let us suppose that ${H^ + }$from each compound is removed. Then, the remaining $Oxide{\text{ ion}}$ $\left( {{O^ - }} \right)$must be stabilized by the Nitro group.
$N{O_2}$ group has both $\left( { - I} \right)$and $\left( { - m} \right)$effect. Hence, it supports the acidic nature of compounds. With the help of $\left( { - m} \right)$effect, the conjugate base $\left( {Oxide{\text{ ion}}} \right)$gets stabilized after removal of ${H^ + }$.
Nitro is an electron withdrawing group. It will withdraw electrons of Oxide{\text{ ion}}$$$\left( {{O^ - }} \right)$$through these two effects. By inductive effect, it withdraws electrons, but it is an effect which is based on distance unlike mesomeric effect.  Thus, the phenoxide{\text{ ion}}$so formed after losing the proton${H^ + }$must be stabilized by resonance. The negative charge present on$phenoxide{\text{ ion}}$must be delocalized into a benzene ring. Nitro group present at meta-position in$m - nitrophenol$, cannot stabilize$phenoxide{\text{ ion}}$as compared to$o - nitrophenol$and$p - nitrophenol$. Also, in$p - nitrophenol$, there is a chance of hydrogen bonding. (Hydrogen from$OH$and Nitrogen from$N{O_2}$group).  So, it is hard to lose the proton${H^ + }$due to$H - Bonding$with$N{O_2}$group. Now, there will be more resonance effect, thus more stabilization of compound$p - nitrophenol$. Therefore these are the resonating structures of$p - nitrophenol$.Thus, in$p - nitrophenol$ there is Stabilization due to resonance and inductive effect too.
Hence, the correct answer is (D).
The most acidic is:

Additional information:
$N{O_2}$ Group do not show mesomeric effect of meta-position in benzene ring. Inductive effect is distance dependent. Hydrogen bonding occurs between Hydrogen, Nitrogen and Oxygen. Hydrogen bonding can be intermolecular and intramolecular.

Note:
While comparing the acidity of the different compounds, always remember the inductive effect and mesomeric effect. The stability of phenoxide ions gives us the order of acidity. More stable theoxide ${\text{ }}ion({O^ - })$, the more its acidity. The order of basicity is just the reverse of the order of acidity.