The correct option is B Ornithine
Amino acids are monomer units of proteins. Amino acids are joined together with the help of peptide bonds to form peptide chains. Peptide chains associated together to form proteins. Twenty one types of different amino acids are found in naturally occurring proteins. Some of the amino acids found in naturally occurring proteins are glycine, alanine, valine, leucine, isoleucine, tyrosine, arginine, aspartic acid, glutamic acid etc. Ornithine is not found in naturally occurring proteins but is produced in liver during formation of urea in ornithine cycle. Liver converts ammonia (obtained from deamination of amino acids) into urea through ornithine cycle.