Which compound represents butanoic acid?

Butanoic acid, also known as butyric acid, is a four-carbon carboxylic acid with the structure CH₃-CH₂-CH₂-COOH. In the context of the given choices, the structure corresponding to butanoic acid features a carbon chain with a terminal carboxyl group (-COOH).

The correct option shows a chain of four carbon atoms (butane) ending with a carboxylic acid functional group. The carboxylic acid is characterized by the presence of a carbonyl group (C=O) adjacent to a hydroxyl group (-OH), which collectively forms the -COOH group.

In summary, the structure indicated in the correct answer represents butanoic acid due to its four-carbon backbone and the presence of a carboxylic acid functional group.