Final answer:
The acidic functional group among the provided options is the carboxyl group (-CO₂H), which is typical of carboxylic acids. It is acidic because it can donate a proton in solution.
Step-by-step explanation:
The acidic functional group among the options provided is the carboxyl group. The carboxyl group has the structure -CO₂H and is characteristic of carboxylic acids. It is acidic because it can donate a proton (H+) in solution, making carboxylic acids behave as acids.
A ketone consists of a carbonyl group bonded to two other carbon atoms and does not have acidic properties. Similarly, an aldehyde consists of a carbonyl group bonded to at least one hydrogen atom and is not typically acidic.
A methyl group (CH₃-) isn't acidic as it lacks a strong electronegative atom to support acidity, and although carbonyl groups (C=O) are present in carboxylic acids, they are not inherently acidic without the associated hydroxyl group (-OH) that forms the carboxyl group.