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

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

Answer: (A)

Verified step by step guidance

1. Begin by understanding what amino acids are: Amino acids are organic compounds that serve as the building blocks of proteins. They contain both an amino group and a carboxyl group.
2. Identify the functional groups listed in the problem: Carboxyl group, Aldehyde group, Ether group, and Ketone group.
3. Recall the structure of a typical amino acid: It consists of a central carbon atom (the alpha carbon) bonded to four different groups: an amino group (\($\mathrm{NH}2$\)), a carboxyl group (\($\mathrm{COOH}$\)), a hydrogen atom, and a variable side chain (R group).
4. Examine each functional group option: The carboxyl group (\($\mathrm{COOH}$\)) is present in all amino acids as part of their structure. Aldehyde, ether, and ketone groups are not universally present in amino acids.
5. Conclude that the carboxyl group is the functional group present in all amino acids, as it is a fundamental part of their structure.