Which functional group is characterized by the presence of a carbonyl group (C=O) flanked by hydrocarbon groups?

The correct answer is ketone, as a ketone is specifically defined by the presence of a carbonyl group (C=O) that is situated between two hydrocarbon groups, which can be either alkyl or aryl groups. This structural arrangement allows for the functional properties and chemical reactivity typical of ketones.

In contrast, an alcohol features a hydroxyl group (-OH) as its defining feature, and does not include a carbonyl group. Aldehydes also contain a carbonyl group, but the distinguishing factor is that in aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom, making it terminal rather than between hydrocarbon groups. Carboxylic acids are characterized by both a carbonyl group and a hydroxyl group attached to the same carbon atom, which is not the case in ketones.

Thus, understanding the structural requirements for each functional group is essential, and the defining characteristics of ketones distinctly set them apart from the others listed.