Which of the following is a nonstandard amino acid?

ANS:D - Histidine

A nonstandard amino acid is an amino acid that occurs naturally in cells but do not participate in peptide synthesis.

Some nonstandard amino acids are constituents of peptides, but they are generated by modification of standard amino acids in the peptide molecule by post-transnational modification.

Example of some non-standard amino acids: L-DOPA, GABA, 2-aminobutyric acid, dehydralanine, selenocysteine, pyrrolysine, N-formylmethionine, Hydroxyproline etc.