In the context of charged amino acids, a basic amino acid has a side chain that contains which functional group that can accept a proton and become positively charged at physiological pH?

• A. A thioether group that is nonpolar and uncharged (e.g., $-S-CH3$)
• B. An amine or guanidinium-containing group that can be protonated (e.g., $-N_{H^3}$)
• C. A carboxylate group that donates a proton to become negatively charged (e.g., $-CO{O}_{-}$)
• D. A hydroxyl group that is typically uncharged at physiological pH (e.g., $-OH$)

Answer: (B)