What is the chemical structure characteristic of a ketone?

A ketone is characterized by the presence of a carbonyl group (C=O) that is attached to two carbon atoms. This specific arrangement differentiates ketones from other functional groups, such as aldehydes, alcohols, and amines, which have different structural features.

In a ketone, the carbonyl carbon is part of the carbon skeleton of the molecule and is flanked by two alkyl or aryl groups, contributing to the ketone's unique properties and reactivity. This structural characteristic is crucial for understanding the behavior of ketones in various chemical reactions, such as nucleophilic addition.

The other options do not describe ketones accurately. A hydroxyl group (from option A) defines alcohols, a nitrogen atom bonded to hydrogen (from option C) pertains to amines, and a carbon atom bonded to three hydrogen atoms (from option D) describes alkanes, particularly methane. Each of these options relates to a different functional group in organic chemistry. The correct answer reflects the distinctive structure that defines ketones within the broader context of organic compounds.