Hint- Higher the electron-withdrawing effect, the higher will be the acidic character

The p- nitrophenol is more acidic than phenol because of the deactivating nature of the nitro group and after losing the protons it makes more resonance stabilization as a nitro group is having strong deactivating nature so it attracts electrons towards it.

Hint: Higher the stability of the reaction intermediate, the faster will be the rate of reaction.

Phenol

+R effect of –OH group enhances the reactivity in an aromatic electrophilic substitution reaction.

Phenols are highly prone to electrophilic substitution reactions due to their rich electron density in benzene ring

The hydroxyl group attached to the aromatic ring in phenol facilitates

the effective delocalization of the charge in the aromatic ring. 

The structure of phenol is as follows:

1. When aniline reacts with NaNO₂/HCl then diazonium salt is formed. When diazonium salt reacts with water then phenol is obtained as a product. 

2. Cumene (isopropylbenzene) is oxidized in the presence of air to cumene hydroperoxide. It is converted to phenol and acetone by treating it with dilute acid. Acetone, a by-product of this reaction, is also obtained in large quantities by this method. The reaction is as follows:

3. Chlorobenzene does not react with NaOH at STP condition. It reacts with NaOH at high pressure and temperature. Thus, in the third reaction phenol is not obtained as a product. 

4. Benzene is sulphonated with oleum and benzene sulphonic acid so formed is converted to sodium phenoxide on heating with molten sodium hydroxide. Acidification of the sodium salt gives phenol. The reaction is as follows:

Hence, option third is the correct answer. 
 

Hint: Phenol undergoes oxidation and form benzoquinone

1. When phenol reacts with CO₂ in the presence of a base then an electrophilic substitution reaction takes place. In this reaction, CO₂ acts as an electrophile. Phenol with CO₂ in the presence of base forms an intermediate, in the next step intermediate is acidified with acid and a salicylic acid or a Ortho hydroxybenzoic acid is obtained as a product. 

The reaction is known as Kolbe’s reaction. The reaction is as follows:

2. When phenol is treated with chloroform in the presence of sodium hydroxide, a –CHO group is introduced at ortho position of a benzene ring. This reaction is known as Reimer – Tiemann reaction. The intermediate substituted benzal chloride is hydrolyzed in the presence of alkali to produce salicylaldehyde. 

The reaction is as follows:

The mechanism is as follows:

3. When phenol is heated with zinc dust then benzene is obtained as a product. The reaction is as follows: 

4. Oxidation of phenol with chromic acid produces a conjugated diketone known as benzoquinone. The reaction is as follows: