I found the exercise on the internet and attached is an image of the molecules.
Ketones are organic molecules that have a carbonyl group with the structure RC(=O)R'. So, the molecules that are ketones will have to present a double bond to a carbon, and that carbon must be also bonded to other two carbons.
So, the only molecule that is a ketone is the molecule C. This molecule is the smallest ketone - the acetone.