What is the primary characteristic of a ketone?

The primary characteristic of a ketone is that it has a carbonyl group (C=O) located within the carbon chain. This structural feature is fundamental to the definition of ketones, which distinguishes them from other types of carbonyl-containing compounds, such as aldehydes where the carbonyl group is located at the end of the carbon chain. In ketones, the carbonyl carbon is bonded to two other carbon atoms, making them an integral part of the carbon skeleton.

The distinction in structure is crucial because it influences the compound's chemical behavior and reactivity. Ketones display unique properties, such as their ability to engage in specific types of reactions that are characteristic of their functional group. Thus, the presence and positioning of the carbonyl group are what fundamentally classify a compound as a ketone, making this option correct.