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

• A. Ketone group ($R-C=O-R\text{'}$)
• B. Aldehyde group ($R-C=O-H$)
• C. Ether group ($R-O-R\text{'}$)
• D. Carboxyl group ($R-C=O-OH$)

Answer: (D)

Verified step by step guidance

1. Understand the structure of an amino acid: Amino acids are organic compounds that contain both an amino group (-NH₂) and a carboxyl group (-COOH).
2. Identify the functional groups listed in the problem: Ketone group (C=O), Aldehyde group (R-CHO), Ether group (R-O-R'), and Carboxyl group (R-COOH).
3. Recall that the carboxyl group is a defining feature of amino acids, as it is present in all amino acids along with the amino group.
4. Compare the functional groups: The ketone, aldehyde, and ether groups are not present in all amino acids, whereas the carboxyl group is.
5. Conclude that the carboxyl group is the functional group present in all amino acids, making it the correct answer.