Which of the following functional groups is present in all amino acids?

• A. Hydroxyl group (${\mathrm{-O}}_H$)
• B. Aldehyde group (${\mathrm{-CHO}}_{}$)
• C. Ketone group (${\mathrm{-CO}}_{-}$)
• D. Carboxyl group (${\mathrm{-COOH}}_{}$)

Answer: (D)

Verified step by step guidance

1. Begin by understanding what a functional group is: A functional group is a specific group of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.
2. Recognize that amino acids are organic compounds that serve as the building blocks of proteins. They have a basic structure consisting of an amino group, a carboxyl group, and a side chain (R group) specific to each amino acid.
3. Identify the carboxyl group: The carboxyl group is a functional group consisting of a carbonyl group (C=O) and a hydroxyl group (OH) attached to the same carbon atom, typically represented as \($-C_1\left(O\right)\left(\mathrm{OH}\right)$\).
4. Understand that the carboxyl group is present in all amino acids, providing the acidic property that is characteristic of these compounds.
5. Review the other options: Hydroxyl group, aldehyde group, and ketone group are not present in all amino acids. The hydroxyl group is part of the carboxyl group, but not standalone in all amino acids. Aldehyde and ketone groups are not typical components of amino acids.