What distinguishes the 20 common amino acids from each other?

The correct answer is C: R-group. The 20 common amino acids share a common backbone structure, but what differentiates them from each other is the side chain or R-group attached to this backbone. The R-group varies in composition and properties among the different amino acids, influencing their overall characteristics and functions. Options A and B are not the primary distinguishing factors among amino acids. Option D is incorrect as the R-group is the key feature that sets apart the 20 common amino acids, contributing to their individual identities and roles in biological processes.