The term "basic amino acid" refers to amino acids that have side chains with a net positive charge at physiological pH (approximately pH7.4 ). Basic amino acids are typically those that contain nitrogen and can accept a hydrogen ion, making them positively charged. Among the options provided, let's consider each amino acid: Proline (Option A): Proline is unique among the twenty standard amino acids because it has a secondary amine group, where the amino nitrogen is bound to two alkyl groups. Its side chain is a cyclic structure. Proline is classified as a nonpolar, aliphatic amino acid and is not basic; it does not carry a positive charge at physiological pH. Lysine (Option B): Lysine has a side chain with an aliphatic terminal (straight-chain) amine group, which is positively charged at physiological pH. This makes lysine a basic amino acid. Arginine (Option C): Arginine contains a side chain with a complex guanidinium group, which has a high pKa and thus remains positively charged at physiological pH. Therefore, arginine is considered a basic amino acid. Histidine (Option D): Histidine has an imidazole side chain, which can be positively charged at slightly acidic to neutral pH. Histidine's side chain has a pKa around 6.0, making it occasionally positively charged at physiological pH, and thus, histidine is sometimes considered a basic amino acid, especially since it can form hydrogen bonds and salt bridges. From the options provided, Option A, Proline, is the correct answer. Proline is not a basic amino acid; it does not have a positive charge on its side chain at physiological pH.