Which letters are not part of the amino acid alphabet?

The amino acid alphabet is composed of 20 standard amino acids represented by single-letter codes. The letters that are commonly accepted in the standard amino acid representation include: A (Alanine), C (Cysteine), D (Aspartic acid), E (Glutamic acid), F (Phenylalanine), G (Glycine), H (Histidine), I (Isoleucine), K (Lysine), L (Leucine), M (Methionine), N (Asparagine), P (Proline), Q (Glutamine), R (Arginine), S (Serine), T (Threonine), V (Valine), W (Tryptophan), and Y (Tyrosine).

The correct choice identifies letters that are not part of this established amino acid alphabet. Within the option that includes J, U, Z, O, and X, none of these letters represent standard amino acids in the sequence mentioned. J and O are not used to designate any amino acids, while U can refer to selenocysteine in specific contexts but is not part of the standard 20 amino acids. Similarly, Z can be used to represent Glutamic acid (E) or