Amino acids vary by side chain at alpha carbon

Steps:

• Identify general amino acid structure: alpha carbon bonded to amino (NH2), carboxyl (COOH), hydrogen (H), and variable R group.
• Recognize that all amino acids share NH2, COOH, and H on alpha carbon, except R differs.
• Note diagram likely shows H as placeholder for R in glycine; other amino acids replace it with unique side chains.
• Conclude changing substituent at H position creates diversity.

Why C is correct:

• The H on alpha carbon is replaced by different R groups to form the 20 standard amino acids, as defined in biochemistry.

Why the others are wrong:

• A. OH: Absent from standard alpha carbon attachments in amino acids.
• B. COOH: Identical carboxyl group in all amino acids.
• D. NH2: Identical amino group in all amino acids.

Final answer: C