Phenol + phthalic anhydride \(\frac{\text { conc. } \mathrm{H}_{2} \mathrm{SO}_{4}}{\text { heat }}{\longrightarrow}(\mathrm{X})\). The product \((\mathrm{X})\) is used as (a) an antipyretic (b) a catalyst (c) an indicator (d) a laboratory reagent

First, write down the reaction formula for the reaction between phenol and phthalic anhydride in the presence of concentrated H2SO4, and heat. Phenol (C6H5OH) + Phthalic anhydride (C8H4O3) ⟶ Product (X)

The reaction occurs through the electrophilic substitution of phenol with phthalic anhydride in the presence of a catalyst, which is concentrated H2SO4. The product of this reaction is 2-hydroxybenzoic acid-1,2-dicarboxylic acid anhydride, commonly known as phenolphthalein (C20H14O4). So, Product (X) is Phenolphthalein.

Now, we will determine the use of phenolphthalein among the given options: (a) Antipyretic: These are substances that reduce fever, but phenolphthalein does not have this property. (b) Catalyst: Catalysts are substances that increase the rate of a chemical reaction without getting consumed in the reaction. Phenolphthalein is not a catalyst. (c) Indicator: Phenolphthalein is a widely known chemical compound used as an acid-base indicator in its colorless (acidic) and pink (alkaline) forms. It turns pink in alkaline solutions and is colorless in acidic solutions. (d) Laboratory Reagent: Phenolphthalein is a laboratory reagent, but its primary use falls under the 'indicator' category. Based on the discussed uses, the correct answer is (c) Indicator.