Which statement about amino acids is true?

• A. There are 25 amino acids.
• B. There are 20 amino acids.
• C. There are 18 amino acids.
• D. There are 22 amino acids.

Answer: (B)

Proteins are built from a fixed set of twenty standard amino acids that are encoded by the genetic code during translation. This is why the statement asserting twenty is correct: the canonical protein-building repertoire consists of twenty amino acids, with the code organized so multiple codons can specify the same amino acid, while still signaling the appropriate amino acid during synthesis. There are a couple of exceptions in some organisms where nonstandard amino acids like selenocysteine and pyrrolysine can be incorporated under special circumstances, but these are not counted among the usual twenty. Numbers like 25, 18, or 22 refer to different contexts or nonstandard residues, not the standard proteinogenic set.