Introduction:
Aspirin, also known as acetylsalicylic acid, is a commonly used medication for pain relief, fever reduction, and inflammation reduction. It is a derivative of salicylic acid and is synthesized through acetylation, which involves introducing an acetyl group (-COCH3) into the salicylic acid molecule. In this case, aspirin is the acetylation product of o-hydroxybenzoic acid.

Explanation:
To understand why the correct answer is option 'A' (o-hydroxybenzoic acid), let's break down the process of aspirin synthesis through acetylation.

Step 1: Starting Compound - Salicylic Acid
The starting compound for the synthesis of aspirin is salicylic acid. Salicylic acid is a white, crystalline solid that naturally occurs in the bark of willow trees and other plants. It has analgesic (pain-relieving), antipyretic (fever-reducing), and anti-inflammatory properties.

Step 2: Acetic Anhydride
Acetic anhydride is the acetylating agent used in the synthesis of aspirin. It is a colorless liquid with a strong, pungent odor. Acetic anhydride reacts with salicylic acid to introduce an acetyl group (-COCH3) into the molecule.

Step 3: Acetylation Reaction
The acetylation reaction involves the reaction between salicylic acid and acetic anhydride in the presence of a catalyst, such as sulfuric acid or phosphoric acid. During this reaction, the hydrogen atom (-H) on the hydroxyl group (OH) of salicylic acid is replaced by the acetyl group (-COCH3) from acetic anhydride.

Step 4: Formation of Aspirin
The product of the acetylation reaction is aspirin, also known as acetylsalicylic acid. Aspirin is a white, crystalline solid with a bitter taste. It is slightly soluble in water but readily dissolves in organic solvents.

Correct Answer - o-Hydroxybenzoic Acid
The correct answer is option 'A' (o-hydroxybenzoic acid). The structure of o-hydroxybenzoic acid is shown below:

H
|
OH
|
C6H4COOH

When acetic anhydride reacts with o-hydroxybenzoic acid, the acetyl group (-COCH3) replaces the hydrogen atom (-H) on the hydroxyl group (OH) of o-hydroxybenzoic acid, resulting in the formation of aspirin. Therefore, o-hydroxybenzoic acid is the precursor for the synthesis of aspirin through acetylation.