The amino acids are mainly different from each other due to the type and nature of:

Explanation

The correct answer is the R-group. Each amino acid has a central carbon atom to which four different groups are attached: an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and an R-group, which is a variable group or side chain that differs among amino acids. The characteristics and functions of amino acids are determined by their R-groups, which can be as simple as a hydrogen atom or as complex as a ring structure. The other options are incorrect because both the amino group and the carboxyl group are consistent across all amino acids and do not vary. The peptide bond is a type of covalent bond that links amino acids together in proteins and is not a distinguishing feature of individual amino acids.