# Step-by-Step Solution: Identifying the Carboxylic Acid Functional Group ## Step1: Recall the Carboxylic Acid Functional Group A **carboxylic acid** functional group is written as: \[ \ce{-COOH} \] or structurally: ``` O || C--O--H ``` It consists of a carbon atom double-bonded to an oxygen atom, and singly bonded to a hydroxyl group (-OH). --- ## Step 2: Review the Answer Choices Let's analyze the options: **A) -OH** - This is a **hydroxyl group**, characteristic of alcohols, not carboxylic acids. **B)** ``` O || C-O- ``` - This is a **carboxylate anion** (deprotonated form of carboxylic acid). **C) OH** ``` | C-OH ``` - This is **not** a standard representation; it seems incomplete. **D)** ``` OH | C - O || O ``` - This represents a carbon with: - One -OH group, - One double-bonded oxygen, - This matches the structure of a **carboxylic acid**. --- ## Step 3: Match to Carboxylic Acid - Only option **D** fits the carboxylic acid functional group. --- ## Final Answer **The functional group for a carboxylic acid is:** \[ \boxed{D} \]