There are 20 different amino acids. what makes one amino acid different from another? different asymmetric carbons

Amino acids are molecules that have a carboxylic acid group and an amine, attached to a specific carbon atom known as the α carbon. There are 20 amino acids, and they are different from each other, in the sense that each has a different side chain, known as the R-group and this R-group is attached to the α carbon.

the answer is d. none of the above