Which one of the following is a heterocyclic amino acid?

(D) Proline # Proline is one of the nonessential amino acids, found in many proteins, especially collagen.> Because the nitrogen atom of its amino group is part of a ring structure (making it a heterocyclic compound), its chemical properties differ from those of the other amino acids in proteins.> It is used in biochemical, nutritional, and microbiological research and as a dietary supplement.