Ketones

Unless I'm mistaken, the R group specifically refers to some sort of alkyl chain or hydrogen.

Although a quick Google says it can sometimes mean nitrogen, halogen or oxygen etc.

so why is ketone the answer here

You agree that a ketone functional group is this:



R must be either an alkyl group or aromatic group. In either case, you're going to have CARBON atoms bonded to the C=O.Wunderbarr is slightly mistaken.
It CANNOT be a Nitrogen or a Halogen or a Hydrogen or an Oxygen. If R was any of these, the functional group has changed; it's no longer a ketone.

If it's a Nitrogen, then it's an Amide/Amine
If it's a Halogen, then it's an Acyl Halide
If it's a Hydrogen, then it's an Aldehyde
If it's an Oxygen, then it's an Ester/Carboxylic Acid.

I hope you do know what those functional groups all are...