Among the groups which make a typical amino acid, which one imparts the unique characterictics to a molecule and to the proteins that contain it?

by Sage
(Houston, Texas)

The groups to select from are as follows:
A. A carbon
B. Amino Group
C. Carboxyl Group
D. Variable R Group
E. A hydrogen atom

I've been unsuccessful retrieving this information from the book or net. I think the answer is Amino Group, but not certain. So I turn to the knowledgeable ones here for assistance. :)
Thank you in advance for your help.
Sage

Read Protein Structure.