Final answer:
The key difference among the 20 amino acids is the type of functional group in their side chains, as this determines their chemical properties and how they contribute to a protein's shape and function.
Step-by-step explanation:
The key difference among the 20 amino acids that are used to make proteins is c. Type of functional group in the side chain. While all amino acids have an amino group and a carboxyl group, what distinguishes them is their variable group, known as a side chain or an R-group. These R-groups can be polar or nonpolar, and they can vary in size, shape, solubility, and ionization properties, giving each amino acid unique characteristics.
The chemical nature of the amino acid side chains determines the amino acid's overall properties, influencing how proteins fold, their shape, and ultimately their function within biological systems. For instance, the side chains of lysine and arginine are positively charged, affecting how they interact with other biomolecules. This diversity in side chain structures allows proteins to have an extensive range of functions and activities within the body.