Here's why:

* Terephthalic acid has the chemical formula C8H6O4.

* Its structure contains two carboxyl groups (-COOH) attached to a benzene ring in a para (opposite) configuration.

* Carboxylic acids are organic compounds characterized by the presence of a carboxyl group (-COOH).

Therefore, terephthalic acid is classified as a dicarboxylic acid due to the presence of two carboxyl groups.