Final answer:
The functional group characteristic of a ketone is the carbonyl group, specifically the -CO- group, where the carbon is bonded to two other carbon atoms.
Step-by-step explanation:
Ketone Functional Group
The functional group that is characteristic of a ketone is the carbonyl group. In a ketone, the carbonyl group's carbon atom is bonded to two other carbon atoms. This differs from an aldehyde, where the carbonyl carbon is bonded on one side to a hydrogen atom and on the other to a carbon atom. The simplest ketone, acetone, has at least three carbon atoms due to this arrangement. Therefore, the defining functional group of a ketone is the -CO- group.