"a question on" indicates: "an issue on The subject of" and as a consequence can only be applied when you can insert the phrase "The subject of" after the "on", though "a question about" can utilized just before anything at all. I like pet animals better than cattle = 'Proper' https://collincedzu.eveowiki.com/1680201/what_does_hoodies_exporter_in_pakistan_mean