In proteinogenic amino acids, what primarily distinguishes one amino acid from another?

• A. The presence or absence of the amino group $-N_{\mathrm{H}}{2}_{}$
• B. The presence or absence of the carboxyl group $-COOH$
• C. The fact that all amino acids have a central carbon bonded to four different groups
• D. The identity of the side chain (R group) attached to the $C\alpha$ carbon

Answer: (D)