Phthalic anhydride reacts with resorcinol in the presence of concentrated H2SO4 to give:

The reaction of phthalic anhydride with resorcinol in the presence of concentrated sulfuric acid (H2SO4) is a classic example of a condensation reaction that produces fluorescein. The concentrated sulfuric acid acts as a dehydrating agent, facilitating the formation of the ester linkages between the phthalic anhydride and resorcinol. The reaction mechanism involves the nucleophilic attack of the resorcinol hydroxyl groups on the carbonyl carbons of the phthalic anhydride, followed by the elimination of water molecules to form the characteristic xanthene ring structure of fluorescein. Therefore, the correct answer is fluorescein.