Read the passage carefully and answer the questions.

Carboxylic acids are the organic compounds which comprise of carboxyl functional group, -COOH. These are generally found in nature, some higher members are known as fatty acids as they occur in natural fats as esters of glycerol. They can be prepared from oxidation of alcohols and aldehydes, hydrolysis of nitriles and amides and Grignard reagents. They are stronger acids than simple phenols and alcohols. but weaker than mineral acids. Substituents affect the acidity of aliphatic and aromatic carboxylic acids. They undergo reactions involving cleavage of -OH and C-OH bond. Carboxylic acids having an alpha hydrogen get halogenated in the presence of red phosphorus to form alpha halo carboxylic acids. The reaction is known as Hell Volhard Zelinsky reaction. Besides these aromatic carboxylic acids undergo electrophilic substitution reactions. They are versatile and are employed in the manufacture of various useful compounds. For example, methanoic acid is used in rubber, textile, dyeing, leather and electroplating industries. Higher fatty acids are used in the manufacture of soaps and detergents. Esters of Benzoic acid are used in perfumery.

Identify the strongest acid

Benzoic acid

The correct answer is Option (1) → Benzoic acid

Acidity of carboxylic acids depends on the stability of the conjugate base. Electron-withdrawing groups increase acidity, while electron-releasing alkyl groups decrease acidity.

Explanation of Options

1. Benzoic acid

The phenyl ring exerts a $-I$ (electron withdrawing inductive effect) on the $-\text{COOH}$ group. This stabilizes the benzoate ion formed after loss of $\text{H}^+$, making it more acidic than aliphatic acids.

2. Phenyl acetic acid

Structure: $\text{C}_6\text{H}_5\text{-CH}_2\text{-COOH}$. Here the phenyl ring is separated by $-\text{CH}_2-$, so its electron withdrawing effect is reduced. Hence less acidic than benzoic acid.

3. Acetic acid

The $-\text{CH}_3$ group has a $+I$ effect (electron donating), which destabilizes the acetate ion. So acidity decreases.

4. Propanoic acid

Has a larger alkyl group $(-\text{C}_2\text{H}_5)$, which shows even stronger $+I$ effect than methyl, making it weaker than acetic acid.