Standard amino acid side chains match the 20 proteinogenic types Steps:

• Identify the general formula for amino acids: H₂N-CH(R)-COOH, where R varies among 20 standard types.
• Recall standard R groups: include alkyl (e.g., leucine: -CH₂CH(CH₃)₂), thioether (methionine: -CH₂CH₂SCH₃), and amino (lysine: -(CH₂)₄NH₂); none has a triol structure.
• Match choices to standards: A resembles methionine's thioether, B exactly matches leucine, C resembles lysine's amine end, D has no match.
• Conclude D lacks a corresponding standard amino acid.

Why D is correct:

• By definition, the 20 proteinogenic amino acids have fixed R groups; HOCH₂CH(OH)CH₂OH (a 1,2,3-trihydroxypropyl) matches none, as no standard R has three hydroxyls.

Why the others are wrong:

• A resembles methionine's -SCH₃ portion in -CH₂CH₂SCH₃.
• B exactly matches leucine's branched alkyl chain.
• C resembles lysine's terminal -CH₂NH₂ in -(CH₂)₄NH₂.

Final answer: D