What chemical group is characteristic of organic acids?

The characteristic chemical group of organic acids is the carboxyl group, denoted as COOH. This group consists of a carbon atom double-bonded to an oxygen atom (the carbonyl part, C=O) and also bonded to a hydroxyl group (–OH). The presence of the carboxyl group is what imparts acidic properties to substances, allowing them to donate protons (H+) in an aqueous solution, which is a fundamental characteristic of acids.

Organic acids, such as acetic acid or citric acid, rely on this specific functional group for their reactivity and identification. The other chemical groups provided in the answers do not exhibit the same properties that define organic acids. For example, CHO refers to aldehydes or ketones, C=O is a part of various functional groups including aldehydes and ketones, and CN refers to nitriles, none of which possess the defining protons and dissociation characteristics of organic acids. Thus, COOH is the clear and correct choice as the hallmark of organic acids.