Which amino acid has a ring structure in its side chain

Proline is unique among the standard amino acids because it has a secondary amine group, where the nitrogen atom is part of a ring structure. This ring is formed by the side chain bonding back to the nitrogen atom of the amino acid, creating a rigid structure. This ring structure significantly influences the folding and stability of proteins by creating kinks in the polypeptide chain and limiting the rotation around the N-C bond. This property makes proline distinct and critical in the formation of turns and loops in the secondary structure of proteins, thus affecting the overall three-dimensional structure and function of the protein. Unlike the other options, which either do not have a ring in their side chain or have different types of side chains, proline's ring directly involves the backbone's nitrogen, making it integral to its structure. Problem: Which amino acid has a ring structure in its side chain? A) Proline B) Arginine C) Alanine D) Lysine