Final answer:
Acetic acid has a carboxyl group as its functional group, which is responsible for its acidic properties and sour taste.
Step-by-step explanation:
The functional group of acetic acid, which gives vinegar its sour taste, is the carboxyl group. In organic chemistry, a carboxyl group is characterized by a carbon atom double-bonded to an oxygen atom and also bonded to a hydroxyl group (-OH), denoted as -COOH. This group is present in all carboxylic acids, including acetic acid or ethanoic acid, and is responsible for the acid's properties, such as sour taste and ability to form ionic salts with bases.