What I want to say is when we are ask
One mustn't talk back to one's parents.
On ne doit pas répondre à ses parents.
Your answer is " On " that is what I don't understand to me the question does not say We mustn't talk back to one's parents. so the answer should be "Un" if not why ?
On can be both: "we" (mostly used in familiar contexts) and "one" (impersonal we).
"Un" doesn't work in French. You can't translate the English "one" literally.
To add to Chris's answer:
Un means one when referring to number or quantity. e.g. Un de mes parents n'aime pas les pommes. (One of my parents doesn't like apples.); Je n'ai qu'un stylo. (I only have one pen.)
On can mean one in the sense that you're using it; it can also mean the rather informal we (in place of nous), as well they or you used in general, and not referring to specific people or groups.
Hope this is helpful.
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