Which chemical group is characterized by the presence of a C=O bond?

The presence of a C=O bond, also known as a carbonyl group, is a fundamental characteristic of aldehydes. In an aldehyde, the carbonyl group is located at the end of a carbon chain, which distinguishes it from other functional groups that also contain a C=O bond.

For example, in aldehydes, the carbon attached to the carbonyl is also attached to at least one hydrogen atom, giving them a specific reactivity profile and unique properties. This structural feature is essential for identifying aldehydes in organic compounds.

On the other hand, while carboxylic acids and ketones also contain carbonyl groups, they feature additional structural elements that define them: carboxylic acids contain both a carbonyl and a hydroxyl group (-COOH), while ketones have the carbonyl group located between two carbon atoms, not at the end of a chain. Similarly, alcohols do not have a carbonyl group; they contain a hydroxyl group (-OH) as their main functional group.

Thus, the defining characteristic of a carbonyl group being at the end of the chain makes aldehydes the correct answer.