Which of the following is a hydroxyl group?

• A. $-{\mathrm{CO}}_{2}H$
• B. $-\mathrm{SH}$
• C. $-{\mathrm{NH}}_2$
• D. $-\mathrm{OH}$

Answer: (D)

Verified step by step guidance

1. Understand that a hydroxyl group is a functional group consisting of an oxygen atom bonded to a hydrogen atom, represented as \(-OH\).
2. Identify the groups given in the problem: \(-CO_2H\) (carboxyl group), \(-SH\) (thiol group), and \(-NH_2\) (amino group).
3. Recall the structure of each group: the carboxyl group contains a carbon double bonded to oxygen and single bonded to a hydroxyl, the thiol group contains sulfur bonded to hydrogen, and the amino group contains nitrogen bonded to two hydrogens.
4. Compare these groups to the hydroxyl group, which specifically has an oxygen atom bonded directly to hydrogen without any carbon or other atoms attached.
5. Conclude that the hydroxyl group is represented by \(-OH\), distinct from the other groups listed.