Identify the structural formula of aspirin.

Aspirin is the common name for acetylsalicylic acid. It is formed by the acetylation of the hydroxyl group of salicylic acid.

Its chemical structure consists of a benzene ring substituted with a carboxylic acid group ($-COOH$) and an acetoxy group ($-OCOCH_3$) at the ortho position relative to each other.

Looking at the given options:
The first structure is salicylic acid (2-hydroxybenzoic acid).
The second structure is methyl salicylate.
The third structure is acetylsalicylic acid, which is aspirin.
The fourth structure is dimethyl phthalate.

Answer: