Esters are functional isomers of :

Options

A. Carboxylic acid
B. Ketones
C. Diketones
D. Diols

Explanation

To solve the question "Esters are functional isomers of:", we will analyze each option provided and determine if esters can be functional isomers of those compounds.

### Step-by-Step Solution:

1. **Understanding Functional Isomers**:
Functional isomers are compounds that have the same molecular formula but different functional groups. For example, an ester and a carboxylic acid can have the same molecular formula but belong to different functional groups.

2. **Option A: Carboxylic Acids**:
- Consider the carboxylic acid propionic acid: \( CH_3CH_2C(=O)OH \).
- The corresponding ester would be methyl propionate: \( CH_3C(=O)OCH_3 \).
- Since both have the same molecular formula \( C_3H_6O_2 \), esters are functional isomers of carboxylic acids.
- **Conclusion**: This option is correct.

3. **Option B: Ketones**:
- Consider a ketone such as acetone: \( CH_3C(=O)CH_3 \).
- The functional isomers of ketones include alcohols and aldehydes, but not esters.
- **Conclusion**: This option is incorrect.

4. **Option C: Diketones**:
- A diketone, for example, could be \( CH_3C(=O)C(=O)CH_3 \).
- Diketones do not have a corresponding ester functional isomer.
- **Conclusion**: This option is incorrect.

5. **Option D: Diols**:
- A diol, such as ethylene glycol: \( CH_2(OH)CH_2(OH) \), does not form esters as functional isomers.
- **Conclusion**: This option is incorrect.

### Final Answer:
Esters are functional isomers of **carboxylic acids** (Option A).