Answer:
a. The alpha-carbon
Explanation:
An amino acid is an organic molecule that contains a carboxyl (COOH) and a amino (NH2) group. It has a central carbon called as alpha carbon.
The carboxyl and amino groups are attached to the the alpha carbon. Apart from them, a side chain denoted by "R" is also covalently attached to it. Side chain varies in each amino acid and it imparts the unique properties to the particular amino acid.