What role does the på play here? Maybe a German explanation could help? I can't see the nuance.
"Hast du mir darauf geantwortet, ob du der Katze Essen gabst?"
Alles klar! Tak!
As a native Dane, i find nothing wrong about leaving out the "på" here. It's probably not strictly correct according to the rules of grammar, but colloquially I doubt many will complain.