Answer:
The correct answer is oxygen
Step-by-step explanation:
Amino acid is the simplest and smallest unit from which a protein is made up of. Amino acid contains carbon, hydrogen, oxygen, and nitrogen. Each amino acid contains an amino group, a carboxyl group, a hydrogen group and an R chain(side chain).
These groups are attached to a carbon atom called alpha carbon. So in an amino acid an amino-functional group, a side chain(R group), and a carboxylic function group are attached to central atom but not oxygen.