Understanding Monocarboxylic Acids
Monocarboxylic acids are organic compounds that contain one carboxyl group (-COOH). They can exist in various structural forms, leading to functional isomerism.

Functional Isomerism Explained
Functional isomers are compounds that have the same molecular formula but different functional groups. This means they can exhibit different chemical properties and reactivities.

Monocarboxylic Acids as Functional Isomers
- **Definition**: Monocarboxylic acids can serve as functional isomers of esters.
- **Example**: For instance, acetic acid (a monocarboxylic acid) has the formula C2H4O2, which is the same as ethyl acetate (an ester).
- **Structural Variation**: The difference lies in the functional group; acetic acid has a -COOH group, while ethyl acetate contains a -COO- group.

Why Not Other Options?
- **Ethers**: Ethers have the general structure R-O-R', where oxygen is bonded to two hydrocarbon groups. They do not share the same formula with monocarboxylic acids.
- **Amines**: Amines contain nitrogen and do not have the carboxyl group, making them structurally different from monocarboxylic acids.
- **Alcohols**: Alcohols contain a hydroxyl group (-OH), which is distinct from the carboxyl group and does not qualify as a functional isomer.

Conclusion
Thus, the correct answer is **option C: esters**, as monocarboxylic acids can indeed function as isomers of esters due to their structural differences in functional groups while maintaining the same molecular formula.