Which of the following is not attached to the central carbon atom in an amino acid?
(a) an amino functional group
(b) a side chain ("r group")
(b) a carboxyl functional group
(c) an oxygen

Oxygen.

Explanation:

Proteins are made of the polymers of the amino acids. Amino acids acts as the monomer of the protein that are linked through the peptide bond. There are 20 standard amino acids.

Amino acids contain the amino group, the carboxyl group, side chain R group and the hydrogen atoms. Oxygen is not directly attaches to the central carbon atom and the oxygen is attached to the carboxyl group. The oxygen is not directly present in the amino acids.

Thus, the correct answer is oxygen.

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.