What distinguishes the 20 common amino acids from each other?

Color is not a characteristic that distinguishes amino acids in a meaningful biochemical context. While some amino acids may appear different colors under specific conditions due to their chemical structure, color does not influence their function or reactivity, making it an irrelevant criterion for distinguishing them.

While some amino acids may differ in molecular size, size alone is not a definitive distinguishing factor. The size variations among the 20 common amino acids do not correlate directly with their specific functions or roles in proteins. Additionally, some larger amino acids can have similar biochemical functions to smaller ones, further complicating the use of size as a distinguishing feature.

This option is incorrect because the R-group is indeed the primary distinguishing factor among amino acids. While other characteristics like size and color may vary, they do not provide the fundamental biochemical distinction that the R-groups do.