Which of the following substances is categorized as aromatic?

Phenol is categorized as an aromatic substance because it contains a benzene ring, which is the defining characteristic of aromatic compounds. Aromatic compounds are generally known for their stability and unique chemical properties due to the delocalized electrons within their cyclic structures. Phenol, specifically, consists of a hydroxyl group (-OH) attached to a benzene ring, making it both aromatic and capable of exhibiting specific reactivity associated with phenolic compounds.

In contrast, acetic acid is an aliphatic compound that does not possess a benzene ring; it's characterized by a carboxylic acid functional group. Formaldehyde, while an important organic compound, also lacks the necessary structure to be classified as aromatic, being simply a one-carbon aldehyde. Butanoic acid, similar to acetic acid, is a straight-chain fatty acid that lacks that aromatic stabilization and cyclic structure. Thus, the unique aromatic nature and properties of phenol set it apart from the other substances listed.