What is the name of the chemical side-group of an amino acid?

The correct answer is the "R group," which is the specific term used to describe the variable side chain attached to the central carbon of an amino acid. This side group distinguishes one amino acid from another, providing each with unique properties and characteristics. The structure of the R group can vary significantly between different amino acids, impacting how they interact with one another and their role in protein structure and function.

The term "functional group" typically refers to specific groups of atoms within a molecule that are responsible for the molecule's chemical reactions, but it is not specific to amino acids. "Variable group" might suggest a similar concept, but it is not the standard terminology used in biochemistry for amino acids. "Peptide bond" describes the covalent bond formed between amino acids when they link together to form proteins, which is a separate concept from the side group of an individual amino acid.