Identify ' $A$ ' in the following reaction. $\text { A + Acetic anhydride } \xrightarrow{\mathrm{H}^{+}} \text {Aspirin + Acetic acid }$

This reaction involves the acetylation of a phenolic hydroxyl group, indicative of aspirin synthesis. Aspirin is acetylsalicylic acid, formed from salicylic acid and acetic anhydride under acidic conditions.

$\mathrm{A + (CH_3CO)_2O \rightarrow[H+] aspirin + CH_3 COOH}$

Salicylic acid has the structure $\mathrm{C_6 H_4 (OH)(COOH)}$, featuring adjacent hydroxyl and carboxyl groups. With acetic anhydride, it undergoes esterification at the phenolic oxygen: $\mathrm{C_6 H_4 (OH)(COOH) + (CH_3CO)_2 O \rightarrow C_6 H_4 (OCOCH_3)(COOH) + CH_3 COOH}$.

Evaluating the options:

Acrylic acid ($\mathrm{CH_2=CHCOOH}$) lacks a phenolic group, so cannot form aspirin. Oxalic acid ($\mathrm{HOOCCOOH}$) is a dicarboxylic acid without a hydroxyl group. Phthalic acid ($\mathrm{C_6 H_4 (COOH)_2}$) has two carboxyls but no phenolic –OH. Only salicylic acid matches the required structure and reactivity.

Therefore, A is salicylic acid.