Let's solve the problem step by step: ### **Step 1: Identify Functional Groups** Look at the structure. The molecule is: - A four-carbon chain (butane backbone). - There is an oxygen double-bonded to the second carbon (C=O) and a hydroxyl group (OH) attached to the same carbon. This is the structure of **2-hydroxybutanoic acid** (an α-hydroxy acid). --- ### **Step 2: Locate All Types of Protons** There are several types of hydrogens: - Hydrogens attached to carbons (alkyl hydrogens). - The hydrogen on the hydroxyl group (-OH). - The hydrogen on the carboxylic acid group (-COOH). --- ### **Step 3: Determine Acidic Protons** The **acidity of a proton** depends on how easily it can be removed (deprotonated). This is influenced by the stability of the conjugate base formed after removal. - **Alkyl hydrogens:** These are not acidic because removing one forms an unstable carbanion. - **Hydroxyl hydrogen (from -OH):** In alcohols, this hydrogen is not very acidic, but in carboxylic acids, this hydrogen is much more acidic. - **Carboxylic acid hydrogen (from -COOH):** This is **the most acidic** hydrogen because the resulting conjugate base (carboxylate anion) is stabilized by resonance. --- ### **Step 4: Select the Most Acidic Proton** - In the structure, the most acidic proton is the one attached to the oxygen of the **carboxylic acid group (-COOH)**. --- ### **Step 5: Final Answer (with Explanation)** **Correct Answer:** The most acidic proton is the hydrogen attached to the oxygen of the carboxylic acid group (-COOH). **Explanation:** This proton is most easily lost because the negative charge left behind after deprotonation is stabilized by resonance between the two oxygens of the carboxylate group. This makes it much more acidic than any hydrogen attached to a carbon or to an alcohol oxygen. --- ### **Summary** - Identify the functional groups. - Recognize all types of protons. - Understand that the carboxylic acid proton is stabilized by resonance and is the most acidic. **Therefore, you should click on the hydrogen attached to the oxygen in the -COOH group (carboxylic acid group) in the diagram.**