Which formula represents an amino acid?

The formula that represents an amino acid is written as R-COOH. This structure indicates that an amino acid consists of an amino group (NH2), a carboxyl group (COOH), and a distinctive side chain represented by R that varies among different amino acids.

The classification of an amino acid includes the presence of both the amino functional group and the carboxylic acid functional group, which are key characteristics. When these groups are combined with a side chain, they produce the diversity of amino acids important for protein structure and function.

The other options do not represent amino acids in the context needed for this question. For example, the first option suggests a generic structure that lacks the distinctive side chain configuration necessary for an amino acid. Similarly, the hydrocarbon formula in the second option pertains to alkanes and does not incorporate any functional groups relevant to amino acids. Finally, the last choice, H2O, is simply water and unrelated to the structure of amino acids. Thus, the choice indicating R-COOH captures the essential components that define an amino acid.